+1 on marking the paper, but I outlined the whole indentation as well as marking the deepest points. That gave me a record of the shape and slant of my sit bones as well as the distance between them.

Then I took the paper saddle shopping. Laying it over a saddle could tell me pretty quickly which saddles I shouldn't even bother trying.

And make sure you're putting enough weight on your butt. You didn't say what you're using for a hard surface, but if you're trying to sit on the floor with your legs extended, your legs will be taking a lot of your weight. A hard toilet seat is a good idea!