I don’t think I have seen specific guidelines for positioning. So thinking about this on the fly, so could be wrong. Love to hear other people’s thoughts.
On leg positioning against brace - I generally sit with my legs in contact with the brace at all time. My feet are looser against foot pegs when I am relaxed, and pushing into the foot pegs to lock me in when I need the full contact for bracing, rolling, etc… In rare case when I am in an uncomfortable boat, I will sometimes (in appropriate conditions) take feet off foot pegs and put them straight out, so then won’t have contact with thigh braces (not an issue with personal boat - more with some of the boats I guide with, as outfitters invariably seem to have old boats).
On where to place thigh brace, I think the brace should be closer to the knee than thigh. The further out toward knee, the more leverage you would get, which is good. The closer in to thigh, the harder it may be to get in and out of boat, so that is bad. But overall, the main thing to look for is comfort. If it is hitting your knee in a funny way and causes pain or discomfort, that is not good.