US-box question

Hi,

last time I was out sailing, something just zapped right into one my brain... as I was fitting the fin, reading the "guide", I realized that I might have misunderstood something about "fin leading edge"...

I have marks from 1 to 4, "wave" to "freeride" saying where to place "fin leading edge". So what part is the leading edge;
-is it the very front edge of the fin, in front of the screw?
-or is it where the "active" resistant part of the fin begins, approx a cm back from the screw?