I understand that 'fitting' one's choice of bike to one's body is a balancing act. Right now I'm working with seat size (I'm closing fast on 76th birthday, weigh 180 lbs, 5'-9" barefoot) with the '23 Tarmac SL-7 I acquired early last September, as an 'upgrade' from the '72 Motobecane Grand Record I'd been riding since early 1972.

First off I mounted a Selle Royale padded seat I'd had on the MotoGR to the Tarmac, but over time & with use I found it too wide as well as a little chafing. Recently I mounted the hard Specialized seat it came with. I just measured that, came away with 145mm at the widest point. It's vastly more comfortable than the SR I'd started with but even with padded undershorts I'm finding it a little too wide.

Using the 'tinfoil method' I've measured my sit-bone spread at 125mm.

At the instigation of a local bike fitter I'm slowly working with changing both seat height as well as seat setback (one at a time) too as I grow more comfortable with how my body fits the SL-7's cockpit. Seat height as of now is 70cm, setback is 64cm (steerer center to widest part of seat), set dead-nuts level with the 'bars at the hoods.

The greater question is: do I use the sit-bone spread # as a hard guide for selecting a more appropriate seat than what I've been using? I don't have a lot of options for trying out seats before buying where I live & ride.

Riding trainer 100% until weather permits a return to outdoor road-riding in (hopefully!) a few more weeks. Trainer's a different experience to what I know of my (strictly recreational) road-riding so whether that should influence my choice of a new saddle I'm welcoming input.