I have no fit expertise but what I did was measure the distance/height from top of original saddle to floor, distance from tip of saddle to bars, and tilt of saddle. Then first positioned the 2nd saddle like the first. But I had to make a small adjustment here and there because the saddles were not exactly the same length and the "sweet spot" was not in the exact position.

Swapping from ISM to Hilo is going to depend on where you sit on the Hilo. The ISM's are supposed to be ridden near the front. The Hilo is a more traditional saddle and you would typically ride sitting further back on the saddle. I ride a Hilo and when in aero end up with a "riding the rivet" position near the front. You'll probably need to experiment with the front/rear location. A starting point can be found by flipping the Hilo upside down on the ISM, matching the saddle side tapers and then measuring nose to stem center or similar. Generally, I would expect your Hilo will end up further forward than the ISM.

The advice above about matching saddle height is good.

do any bike fitters offer fit expertise for saddle type and position only, as opposed to the full fit (assuming you like the rest of the bike position)

Compare the instructions for the two seats. I bet there's an advised position for each that could give you a relative starting point.

When you say you tried the Cobb, realize he has a full spectrum of saddles. At my last fitting, he had me try 4 or 5 before I found the Randee which was an instant fit. Call them, order a couple and send back what you don't like within 90 days. Decent odds you'll talk to John himself.

You don't need to pay anybody to ''professionally fit'' you on a new saddle. You already know the saddle height, that shouldn't change. You just have to figure the fore/aft and the tilt. Do you have a significant tilt on the Adamo? It looks like the Bonti saddle is made to be ridden level. I would start level or tilting very slightly. Next you have to figure where they wont you on the saddle. Where are you on the Adamo? Mount the Bonti so you will be on about the part. Now put the bike on the trainer and adjust until it feels right and when you get out on the road, take an allen key with you and make your last adjustment and you should be right on.

It's a lot more fun to do it yourself instead of having the ''professional fitter'' guess where the saddle should be. Or if you want, you can pay him and then make the final adjustments when you ride outside anyway.

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''Sweeney - you can both crush your AG *and* cruise in dead last!! 😂 '' Murphy's Law