If you reduce the length of the back fork, you reduce the depth of the seat. So, don't tinker with the fork - place all your adjustments below that point.
Shaping the underside inseam between the existing fork and the knee causes the trouser to get closer to the back leg (i.e. a slimmer "cut"). So, if the blousing is under the buttock, this is the adjustment you want.
The saggy bottom problem in the photo is as SG said: front rise too short; back rise too long.
Work one up in a muslin to observe.
Would you leave the topside inseam between the fork and knee untouched? If you don't, what is done if the inseam lengths become different for non shaping cloth? I remember reading a thread about easing the inseam, and someone was mentioning a longer underside inseam and a short topside inseam that I believe was then shaped to be longer. I want to make my first trousers in some sort of cotton twill, so I'm not sure what I would have to do with a longer underside inseam, as I like my pants with a slimmer thigh.
Edited by Che Pasticcio, 28 November 2010 - 04:42 PM.