I think you mean bottom of the thighs and top of the shins? Yes, 1/2" then try it on again. If necessary trim a little more.
But first, if you're trimming the thigh tops (at the front) why not just trim ALL the way around the top of the thigh? As long as you match the original shape, it should be fine. If you plan on putting a return edge there, make sure you leave 3/8" extra plastic. This will reduce the length of the thigh at the tops, then you can leave the bottom of the thighs as is (no notches required).
In regards to the hand plate trimming, check this thread here:
http://forum.whitearmor.net/index.php?showtopic=13491