RE: Lessacher Duo 40 for F-Type 148 - Sail Sizes?
If your fin fits flush when you tighten the front screw, it sounds like the
angle of the overhang is all right, but the Tuttle head may need a little "shimming" to get your fin to fit with the overhang flush but having both the front and rear taper radius's fitting a bit more tightly.
It's like the font angle and the overhang are working to hold the fin perpendicular, but the rear taper radius is either not mating correctly, or there is something slightly off with the angle of the rear taper radius.
Might be a good idea to put some packing tape "shims" on the rear taper radius and see if that doesn't tighten up the rear so you can put the same torque (not very much here) on both screws. I'd suggest putting more tape at the fin blade end of the root to kinda push the fin forward in the box and rotate it a little more toward the front of the board.
If you use this fin in other boards, the shim can work.
If this is the only board you use this fin in, you could replace the tape shim with a thin layer or Marine Tex or JB Weld Quik and then refit your fin so it fits correctly with both screws torqued the same.
Hope this helps,