Depends on board size. Not as critical as on a kitefoil board since you are strapless
Based on my readings, it is actually not a fixed position, its is a range of positions around 25% of board length.
If the board bottom is not flat, you can move the plate fore or aft this quarter length position by 2 to 3%
The middle of the plate (also mid chord) to tail edge distance
Mid plate position = (board length /4) +/- 2%
Example for a 1.9m board: 1.9/4 = 48cm +/- (1.9 x 0.02) so plate position is 44 to 52
8’ - 22" = 60 +/- 5 cm
7’ - 20" = 53 cm +/- 4cm for 2.1m board
6’ - 18" 1,80m -> 45 +/- 4 cm
5’- 16" kitefoil board 150cm = 38 +/- 3 cm that is 35 to 41 cm
Then, be careful how you drill the board to mount a kitefoil or SUPfoil hydrofoil because 4 holes will create weakness points.
Best and cheapest method (Credit Aventout on the French Kitefoil Forum):
1- drill 4 holes (usually on a 165 x 90 mm pattern) with 9mm bit through board top and bottom
2- Mount a Allen wrench with 12 or 13 mm shorter side on a drill . Empty inside each 12x2 = 25 mm diam hole with Allen wrench between top and bottom skin without removing the skin which is essential for future strength. Holes bigger than 28-30 mm in diameter could make the board fragile.
3- fill holes with epoxy resin and micro balloon mix or resin plus fibre cuts with slow hardener (to avoid exothermic reaction that will melt the XPS inside the board)
4- wait until set/cured (could take 2 days) and drill with 8mm bit (not 9mm) for 8mm screws and bolts (A4 316 Stainless Steel)