My favourite 'go to' reference is "Fitting and pattern alteration: A multi-method approach to the art of style selection, fitting and alteration" by Liechty, Rashband and Pottberg-Steineckert. Or the big red book.
Above are three ways to add width to the top of the leg. This book always offers a number of ways to tackle a fit issue. I've used the slash method (see below).
|Close up view|
How did I figure the amount to open the leg? I added 1 cm to the front and the back. This then gave me 4cm extra width for my thighs. That's a lot of extra thigh room.
This shows you the amount I added for the sway back and the deeper curve on the back piece. The first test shorts were 4cm too low at centre back so that's how I knew how much to insert.
The book shows you the initial image with the drag lines so you can analyse what adjustments you need to make, if it's just you, a camera and a family member on hand.
If I stop cycling and training, I might need to read about a 'low buttocks curve' adjustment.