Why are you using a 4.2 m2 rig in 15-20 knots of wind on such a big board.
At 75 Kg the board is pretty big, but your 4.2 and 5.2 m2 Rigs are way too small.
For your stated wind speeds, one would expect you to be on 6.5-7.5m2 rigs.
You could use a smaller fin, but the 48 cm you have should carve jibe very nicely
if you ever got going fast enough.