I feel your pain here Greg, as a fellow skinny chicken leg trooper lol.
I trimmed the front haves first so the cover strip covered the front ridge on the two halves. My cover strips are 20mm wide so left 10mm of the ridge on each half at the front. I then tape them together and while wearing my under suit put my leg through each thigh and slid it all the way up as high as it would go. The next step is easy if you have a second person helping. Overlap the two halves at the back and mark a center point of the overlapping halves, both at the top and bottom of the thigh. Tape them with those pencil marks aligned and have a look from the front to see if you have a nice taper in the thigh shape. What you want is a taper in the shape (wider at the top and narrower at the knee end. Take the thighs off and have a look, if you are happy with the shape draw a line from the top pencil mark to the bottom on each half and that is your trim line.
What I did from there was add an extra 10mm plus per side out towards the edge and create a new trim line and cut the excess material away. I did this so if something went wrong the cut the thighs were slightly larger and I could re adjust and re trim if needed.
Hope that makes sense, usually I have a set of armour here I am building for someone and I would quickly mock it up and take some example photos, but currently I dont sorry.