I measure just at crotch level. I think what matters most is that you are consistent. I'd just choose the biggest part (which for me is where I measure).
I use upper (widest part), mid, and lower (2-3" above knee). I find that for me 3 measurements are better since the upper and lower thigh are slower to change than the mid-thigh.
Being consistent wih the spot is definitely the key to monitoring changes.
I don't know if there is a "correct" spot, as long as you do it consistently each time. I would say just pick a spot, and try to measure there each time. Others may know of the "right" way to do this, and I'm sure there is some research on it somewhere out there.