IMO it's best for you to go try one on in the store. just too much room for error in a very expensive belt. be sure to leave one or two extra holes for low rise pants or in case you think you might ever need the extra room. i always forget to do this and then wind up stuck LOL.
wow, i must be rather hippy since i need a 90 at a 29/30 and i don't like my jeans that low. you really do need to try them on, it could be an expensive mistake! i'd probably go with an 85 if i were a 28.
size 25 7 for all mankind size 80 but my TR jeans are lower rise so I think I would wear those with 85, best bet try belt at store with favorite pants/skirts, bring them all to the store.... ( I have jelly thighs..........)