I see all these suggestions online for how to measure one's sitbone width at home, but I've never/rarely seen this one detail clarified: does one measure inside-inside, center-center, or outside-outside?

Specifically, I'm interested in measuring for leather saddles...I have good experiences with plastic-based saddles with a listed width of 143mm, and a leather saddle listed at 160mm. Another leather saddle listed at 146mm ended up being too narrow (I found myself sitting on the crescent part of the frame), but I'm wondering how to determine the narrowest I can go with a leather saddle. I know I need to account for a bit of extra width so that I'm not sitting on the frame, of course...