First, I'd like to thank all who commented on the fitting issues for my pants. I did look at Debbie's site and somehow thought it would not be the "one" answer to my problem, as I've become convinced that there is no one-step solution for me. But I think I have found part of the solution there. Tonight I spent adapting my pattern and making a muslin.
My starting point, after reading all the information was the Threads issue of January 2006, an article by Joyce Murphy Adjusting pants from waist to seat. In this article she describes "body space" as an important point in fitting pants. And it does make sense to me, as women have very different shapes. One needs more space in the front, and others (like me) more in the back.
The picture above shows the body space in my pattern, which is 15 cm. I tried to measure my own bodyspace by taking two rulers, and it is 19 cm, which means that 2 cm more is needed (half of the extra width in the pattern). The article describes how this must be done after pin-fitting the muslin, but as I have no one to help me with that, I tried to do it in this more theoretical way, by measuring.
In this picture I tried to show how I measured this (alone in the house, so no one to take a better picture). Standing with by back to a wall, one ruler to the wall, and the other over my front, meeting in a 90 degree angle. My ruler has white space to start with, so the actual size is 19 cm.
The adjustment that follows from the article can also be seen in the first picture. The 2 cm is taken from center back, and added to the side seams.
After this adaption I made my muslin, and this was the first result.
Not too bad, the diagonal wrinkles are gone. The white is waistband interfacing, that I added to get a good impression of the fit with waistband.
This is the result of stitching the crotch a bit deeper. Not a great improvement.
And this is the result after pinning extra width away. (Did you ever try to pin pants you are wearing on the back? Not a great experiment!)
This is where I stopped tonight. This looks much better, and I think I now need the fish-eye adaption on Debbie's site. But that is for Friday.