Ummmm..... no, shorten your harness lines so that you stand up taller and more OVER the board.
This should make things so you cannot push SIDEWAYS as much as you do on your formula board.
I did not say the body needs to be "near the fin". What I mean is that your weight needs to be more inboard, and pushing down (vs sideways).
I think the fin box and rear footstrap arrangement (you are using two rear straps.... right?
will put your back foot just over the front of the fin, which is about right.
What you want to do, is keep the pressure DOWN on the fin to keep it in the water, but put less sideways (across the top of the water to the lee side) pressure on the fin so you don't push it loose and spin out.
When you are going fairly slow, you can't push hard because there is not enough water velocity past the fin to take very much side pressure.
When you are going much faster, you can increase the pressure some, but never pushing hard like on formula with a 70 cm fin.
Hope this helps,