Could we call it a FBumA (playing on the full bust alteration formula)? It would have similar alteration issues (opening space between seams).
When you're adding to a pants pattern, the back piece is all you have to deal with. If it's a jeans alteration, the seam between the upper yoke and the main back piece IS the dart you're altering on. However, let's just stick to trouser style for the moment.
Assuming you started with a pants pattern that was perfect enough in the waist (A) and thighs, just needed more in the back, adding width (B) and a dart (red v-shape) above that width gives you more in the thigh and leg (C) than you wanted.
I'm not fond of slashing entire pieces by length to get what I want, so if I could avoid it, I would.
Version D is throwing off the grain line on the side seam and that's going to get weird in a hurry.
So we go towards the back center line with E.
I have nipped off a little at the back waist and the side hip, because that is what I end up doing. I am bigger in the back than the front and I need all that width through the meaty part. Essentially all I've added in the crotch is the equivalent of a gusset (which has been an alteration staple of my pants wardrobe for years) while adding who knows how many inches across the back. I could make the back crotch deeper as well, lengthening the center seam upwards to make the seam longer
I confess I work the other way; identify the sizing from the butt and taking in the other parts. And I always have issues with how pants fit through the waist in the back. I am reconsideing this in light of reading about pattern adjustments; getting the fit right through the shoulders is trickier than getting the bust to fit. But you'd buy a blouse pattern based on your chest measurement, which would tend to throw the rest of it off.
Perhaps the moral of this story is: you gotta make a muslin first.