So after several sessions strapless I was confident where to put the extra inserts, they go so that the tip of my toes is almost level with the end of the rubber pad. I added them in between the outboard and inboard rows.
Tested today in very light wind with the 1400AH wing, foil mast all the way back, sail mast foot 2 thirds forward: the set up was balanced.
Compared to strapless it is easier to get going quickly without moving the front foot around. Once locked the front, it is easier to move the back foot and pull on the harness, much better when going upwind.
Changing wings and sails should be possible to balance the rig just by adjusting the sail mast foot.