Join Date: Dec 2008
Location: I am omnipresent (mostly Alberta...)
I can't really tell you how to measure your butt, but I can give you some tips!
If you can, try sittin in saddles with different seat sizes (that you know the size of!) and see which one you can fit a closed fist behind your butt in.
I would reccomend getting a saddle that fits you right now, rather than trying to find one that will fit your future bum. Unless your seat size changes drastically, you'll just have a little extra room in the saddle seat, which is almost never a bad thing!