Let's start with, how do you think you would do it? Not trying to be snarky--I'm not sure there's a bullet-proof way to do it, but we can start with what make/model you have, what you think you might be porting over to, and deal with any potential complications as needed. -J

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Life is tough. But it's tougher when you're stupid. -John Wayne

Get a reference for height: measure from top surface of saddle to center of bottom bracket.

Get a reference for fore/aft: measure from center of headset to either the saddle tip (easy, but won't work if switching to/from an ISM-style saddle) or the spot where your sit bones contact the saddle.

That will put you in the ballpark. Different saddles are going to do different things to your position, but that's where I'd start.

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Your favorite mafia sucks.

I was always taught nose to stem but with all the different designs now of days I'm not so sure. I may just break down and go to my fitter but they're like way out of the way for me.

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I still lapped everyone on the couch!

Also I'm on a stock Felt saddle right now. Looking to get something better. Have an old specialized tri saddle that's pretty good but its been a while and I want something new.

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I still lapped everyone on the couch!

Briefly, you want to figure out where you sit on the current saddle, and where you're going to sit on the new saddle, and install the new saddle so that the point you're sitting on it matches up with where you used to sit on the old one.

Example: Switching from a conventional saddle to a Specialized Power typically means the nose of the seat will be 3-4cm further back on the Power.

Switching from a conventional saddle to a split-nose saddle such as a Cobb JOF or an ISM means the nose of the seat will be 5-9cm further back on the split-nose.

Of course, this assumes that you had a good position on the old saddle. If your position was sub-optimal to start with, then you may as well just work with a good fitter to find the right seat and get it in the right place.

It's all about the sit bones as others have pointed out.

I switched to an ISM road, and as vjohn mentioned, the saddle have to be placed further back, because your sit bones will rest kind of in the middle, which is different than in a conventional saddle.
Take the time to read the vendor specs of the one you are going to use.

I asked a similar question recently, and was pointed to the below resource:

https://docs.google.com/...id=0&single=true

That's a cool resource, thanks for posting! I would speculate though, that the point each person sits on a saddle is going to be a bit individual. Some may be closer to the 75-80mm width point, others at 95-100mm. Thus my suggestion to find where you sit on the seat now, and attempt to replicate that.

When I firt started playing with the Power Saddle I simply placed it upside down over the Phenom that I had been using, and moved the Power until the widths of each seat (as viewed from above) lined up, then I simply measured the difference from the tip of one saddle to the other to know how to initially set it up. That "90mm" spreadsheet seems to be doing something similar.

That upside down saddle trick is a good idea. I think I may have done that as well when I changed to my ISM, but never thought to codify it. It's not perfect because shapes can be really different, but I think it got me within a cm on the first try to where I ended up.

What we have so far:
1. Make an estimate of the nose fore-aft differences for where you're going to sit using the handy spreadsheet or id'ing the sit point and measuring to the noses.
2. Measure saddle height to that sit point from BB and position from nose to bars (my pref), or something that gives you horizontal position. Write down everything.
3. Once you're sure you can put your old saddle back in the same position, then you can take it off.
4. Put on the new saddle using the height to sitpoint and any necessary fore/aft adjustments.
5. Make small changes (mm's) to tilt, fore/aft, height to dial in the rest of the way.

The first 4 shouldn't take too long. The last one might take a half hour of riding/tweaking to get it dialed in just right.

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Life is tough. But it's tougher when you're stupid. -John Wayne