G'day

Got a weird question here. I've been getting faster ever since I got myself a GT-31 and started focusing on speed. But I've found that above about 25kts my front foot starts slipping out of the straps in flat water.

Any tips for front strap technique?
Tighter or looser (deeper foot)
Forward or back (on the board)
Inboard or out (I've set mine at the rail)

And does mast base position or boom hight have any impact?

Cheers
Berowne.