Pants fitting has been an ongoing struggle for me, as it is for just about everyone who sews their own clothing. It doesn't help any that so many of the 'experts' seem to offer diametrically opposed advice, including some absolutes like 'never scoop!'
I'm sorry, I just don't think there are any such things as absolutes in fitting. There's waaaayyy too many variations in shape of the human female form. Since I've become, um, highly concerned (that does fall a bit short of obsessed, you know) with fitting, I've become a people watcher. There are women with huge cabooses and there are women who must surely sit absolutely on their hip bones. If a pattern was originally drafted for a flat seat, with very little curve in the back, simply adding on to the crotch point and the waistline is not going to correct for a fanny that curves. Ya gotta scoop it out.
Now, I don't have an excessive backside, but I apparently do have more rear curve than the average sewing Jane. My common sense told me that I needed to scoop the rear, but it didn't work. The resulting pants were always too tight at the low hip. I didn't want to add to the side seams; I had visions of jodphurs. So I kept adding to the ends of the back crotch seam; it worked...sort of.
Then arrives issue 122 of Threads. In it is an article on pants fitting. An excellent article, I might add, with wonderful visual aids (I'm wondering who made those teeny pants for the wooden doll models, but I digress). The author writes about scooping the backside and adding the amount removed to the side seam. Suddenly, I saw my error. I had been removing fabric from the curve itself; the recommendation is to remove the fabric from the curve all the way up to the waist..then add that whole bit onto the side seam. Bingo! No jodphurs! The addition goes all the way into the waist!
I think I've found my answer. Now, to find some time to make another pair of Oxford Pants and try it out!