First up, we'll look at my dressage saddle since that's the saddle I primarily ride in.
So I apologize that these are not taken from the best angle, but I'm working with what I've already got. You can see that my knee fits "into" the knee block, which is how I think a dressage saddle is intended to fit. The knee blocks are there to "hold" your leg in place, and an improperly placed knee block can seriously mess with your leg position.
If I was to take my feet out of my stirrups I would guess that the stirrup falls right around my ankle. I was always told that dressage stirrups should be long enough that when your legs are hanging at rest, all you should have to do is "lift your toe" and be able to slide it into the stirrup without much (or any) bending of the knee to get there. Just my two cents.
My leg extends down a few inches below the flap, but because it's a dressage saddle, the flaps are super long. Plus I have midget legs, so I don't know if this is a helpful comparison for most :) Overall, I really like how I fit in this saddle and have never had anyone comment otherwise.
Now onto my jumping saddle, which is what I think people are discussing more frequently.
So the jumping picture isn't idea because it's a 3/4 angle, but I think it serves the purpose. I would say I have about an inch of space between my knee and the edge of the flap... but maybe a little more. I am extremely comfortable with this "set up" of saddle/flap/combo and feel very secure in it, which I think is evidenced by my leg position in each picture.
The bottom of the flap hits me about mid calf, but again I would point out that I have midget legs, and extremely short calves. So while it may not be the best for comparison, it's all I have to work with :)