Step one: pin out the extra fabric.
It's an awful lot. When I undid the pinning (having replaced the pins to outline what had been pinned out) this is what I was confronted with.
It's roughly 2cm of length below the back waist, and 2.5cm at the widest point of each under-seat fisheye dart.
Step two: draw that on the pattern piece (at right). Hmm ... that was strangely easy. But now what?
Step three: think. How the heck do I get those adjustments to the outer edge? Photo at left was an effort to see if I could use the technique that Kenneth King wrote about in issue 102 of Threads Magazine. Basically, you measure the pinned out dart at each line and then take that amount out at the other end of the line you have drawn perpendicular to the CL of the dart. The result here (red lines show the adjusted seam lines) didn't fill me with confidence, since it would have the effect of taking horizontal width out of the CB which I am pretty sure wasn't part of my problem.
Step four: research. Good old Google. I plugged in the term "flat seat adjustment" and up popped (among many other sites) the brilliant Flickr tutorial authored by the brilliant Ann Rowley.
All of my faithful readers will no doubt recognize this elegant lady, the winner of the first go-round of the Great British Sewing Bee. She is a talented knitter as well as a fabulous seamstress. Her photo exposé on the making of Vogue 8804 (the Chanel-ish jacket, my version of which is languishing for a second summer in a UFO closet of shame) beautifully illustrates all 94 (94!!) steps from the pattern instructions. I bow down to Ann Rowley, truly.
But I digress. Her flat seat adjustment instructions are oh-so-simple and, miracle of miracles, they end up producing precisely the result I am pretty sure I need.
Step five: Slice, dice and adjust. Mark one cutting at right angles to the straight of grain, through the crotch point; one running up through the waist, parallel to the grain; and one at an angle intersecting with the crotch curve.
The result is at right. Black marks the cutting lines, green shows how much they are overlapped. Blue shows the original pattern piece outlines and red shows the adjusted piece. As you can see, the crotch curve is lower and the CB seam is more vertical (less darted). The back waist is lower and the side seam more curved.
I followed Ann Rowley's instructions to the letter, including the amounts she said to adjust. This had no automatic relationship with my particular figure, but it so happens that the distance between the more or less horizontal green and black lines (the fisheye dart in action) is 2.5cm, which is exactly what I needed to remove as per my pinned out darts. The adjustment also narrows the back waist somewhat, which I needed to do anyway, so I will not add this back at the side seams.
The question remains whether I need to take more length out at CB. I will try sewing this pattern as adjusted to this point, having taken Ann Rowley's words of wisdom to heart:
"The suggested size of the diagonal overlap may be adjusted but even so you're unlikely to get a totally flat smooth seat. Do remember that you need to bend, stretch and most importantly, sit down! Don't be tempted to over fit ."