I don't know why, maybe it was trip to Oklahoma last year, but I have been more concerned about snakes in the last year and a half than ever before.
In researching snake boots/gaiters/chaps the consensus of opinions I have seen indicates that the VAST majority of strikes are in the ankle/shin area. Very rare cases above the knee from what I have read, heard and seen. Granted, all my information came from the internet.
I'm not trying to convince you that what you want to do is wrong, far from it. Just presenting a little research tidbit.