I was hoping in seeing how the foot pocket rest on my foot in the best position, you could see how it should fit, doesn't matter if I am in boots or barefoot, the fit must be the same, you see in the yellow fins (foils) I have the comfort instep inplace but not on the black fins (Pros), since the Pros are a size smaller. I will try and get some more photos tonight and show a couple of different angles.
I believe the best fit will occur when the ball of your foot (widest part) should slightly beyond the front of the foot pocket.
Alternately, imagine a pair of dress shoes with laces, the 5 lace hole kind (ie wing-tips), the foot pocket should rest in the same place that the laces would be.