Wrap the chain around the big / big without passing through the derailleur. Measure the shortest chain length that will fit, and add 2 links. That's for a standard short cage derailleur.

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Cool, thanks. I'll try that method and maybe leave an extra link to play it safe, then remove if not needed.

You don't need the extra link.

The big-big plus two is perfect.

No, don't do it that way.
The new road derailleurs run longer cages now and require longer chains.
Put the chain around big big combo and shorten it to as much as you would like to see the derailleur stretched out.
Make it on the long side as it is easy to take out links later and generally chains run better long.
Take into account the interaction of B screw adjustment.
Join the chain with an old quicklink.
Now shift through the gears and see if the chain goes slack or the chain touches itself in any of the gears that Di2 allows you to run.
If it doesn't go slack, check the clearance of the top pulley in all gears and see if pulling a couple of links together helps or hinders it in keeping as close as possible without interference in any gear it will run.
If it helps, shorten it, if not leave it be.
Again, make sure you take into consideration B screw adjustments.

You will eventually work out the perfect length for your setup, not simply one that stops the chain exploding the rear derailleur.

Ditto others. Don't add links. The big/big+2 is the standard on both Shimano's and SRAM's websites for chain sizing. I have sized multiple chains like this. And, I have sized incorrectly too. Incorrectly sucks.

Yeah, with a long cage I'd add an extra link (optional) although whether you do so depends on what range you're running and how much chain you need the derailleur to take up.

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That's pretty much what I do as well

One of the things that Shimano have always excelled at is service instructions.

http://si.shimano.com/...0-02-ENG.pdf#page134

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That's how I used to do it before I started experimenting with a 1x setup (which I've now ditched). The 1x seems to like a bit more chain tension. So I use the big big +2 now. The other piece is that I use my biggest cassette that will go on the bike.

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I learned this way (the link I posted) after installing a OSPW system. I orginally cut the chain too short which caused various issues.

Am I right in assuming that the big big +2 won't work for a OSPW system?

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I wouldn't think so.

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You should be able to change to regular shift mode by double tapping the button on the junction box.

Clearly the big/big + 2 is recommended approach. It works and is the official instruction from Shimano. When in doubt, do that.

But what happens if you swap cassettes/chainrings to a bigger size? Go from a 25t cog to a 28t and sudenly you're a link short:(. ...so, I don't see the problem with adding an extra link if that's a possibility.

Another approach that I've used without issue is to fully instal the cable through the derailleurs and then (in small/small) take out all the slack from the chain to the point that the RD pulley is under spring tension and use that as my chain length guide. This would lead to the max chain length. It works fine. I know it's not the recommended approach, but I don't notice any ill-effects (no chain-slap, no poor shifting response, or chain drop) as a result.

Shimano has never recommended the big big plus 2 method.
That is a SRAM method and their derailleurs function differently from Shimano in that the top pulley is also the cage pivot.
Shimano generally has the pivot offset from the cage although MTB changed this in recent years and road is heading MTB way so as to accommodate wider range clusters.
Shimano method was always make sure you are within maximum tooth count and then set the deraileur cogs vertical when in small front and large rear.
They have never ever spoken of add two although you will find mention of add one to three links in the latest instructions.

The correct way for all systems is to find the length that gives a consistent top pulley height in all gears without going slack in running gears or stretching the derailleur too far.
You may also want to consider if you are going to be running different cassettes as well.
Sometime this can be accommodated well and some other times it will compromise shifting.

Never? hmmm... let's see

http://si.shimano.com/...DM-CN0001-05-ENG.pdf

...Page 9

"For triple/double front chainrings
1. Mountthechainontothelargestsprocketandthe largestchainring.
Next, add 2 links to set the length of the chain. "



This is for the 11-speed instructions. I don't see any mention of 'add THREE links'. Again, this IS for the latest instructions.

...I think it's time you take another look at Shimano's recommendations.;)

http://si.shimano.com/...0-02-ENG.pdf#page134

*vertical when in large front and small rear. (I'm looking at the CN-7901/6701/5701 instructions right now for reference).

I prefer the little-little-with-no-slack method because then any cassette size, as long as its within the spec of the rear derailleur, can be installed. I don’t like the idea, for example, of sizing a chain to a 28t cog if the RD can handle a 30t or 32t. If you do, at some point down the road a 30 or 32 could get installed and rip off the RD.

Lol, I did almost exactly that at IMMT this past Summer. Decided I'd throw a 32 on but never thought to put on a longer chain. Shifted into it while riding the day before, rd jammed big time into a kind of horizontal position, took me about 10 minutes to get it free.

I've always preferred to swap chains when swapping out cassettes anyway. Sram master link makes it easy, and now I've gone to hotwaxing the chains so I'm taking it off occasionally anyway, and the chain doesn't get my hands greasy anymore.

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2020 National Masters Champion - M50-54 - 50m Butterfly

[/quote] Another approach that I've used without issue is to fully instal the cable through the derailleurs and then (in small/small) take out all the slack from the chain to the point that the RD pulley is under spring tension and use that as my chain length guide. This would lead to the max chain length. It works fine. I know it's not the recommended approach, but I don't notice any ill-effects (no chain-slap, no poor shifting response, or chain drop) as a result.[/quote]

This is what I have used on over 20 occasions and it always works. It makes sense to have at least some tension on the RD spring when in the shortest combo, ie small/small.

Another approach that I've used without issue is to fully instal the cable through the derailleurs and then (in small/small) take out all the slack from the chain to the point that the RD pulley is under spring tension and use that as my chain length guide. This would lead to the max chain length. It works fine. I know it's not the recommended approach, but I don't notice any ill-effects (no chain-slap, no poor shifting response, or chain drop) as a result.[/quote]


This is what I have used on over 20 occasions and it always works. It makes sense to have at least some tension on the RD spring when in the shortest combo, ie small/small.[/quote]
That's the way I learned to do it back when... Well let's just say the rims weren't wood anymore, but 10 speeds ruled the day. I was born doing it that way an I'm a gonna die doing it that way and no sidewindin, bushwackin, hornswagglin' cracker croaker is gonna make me do it differnt.

I've done that before and the only problem i have with it is depending on the derailleur and cassette, you need to have enough chain tension for the top pulley to clear the big cog.

It's possible to need to take another pair of links out but that's on a rare occasion