This is a great topic and one I have experimented with over time. I've discovered it's all about distance from front strap to mast plate, and the back foot really doesn't matter if the front foot is correct. For me, it's usually a 60cm stance although that changes depending on if I'm toeside and /riding upwind. Wider seems really tiring.
Zeeko recommend 37cm from bolt to bolt (rear bolt front strap to front of mast plate) and whenever I deviate from this, performance and balance seems to suffer.
Before foiling I only rode strapless SB - never TT so I'm pretty comfortable that my SB technique is solid. I know I move my back foot around a lot depending on what I'm doing. Upwind toeside = narrow stance - say 12" or 30cm, back foot right over in the upwind rail, and a much wider stance when I'm on the wave. I hate riding with straps on SB now because it locks me into the "wrong" stance for the entire session. Do you set up for upwind ease or on the wave ? One angle will be perfect, the other direction will suffer
I do exactly the same on the foil - As long as my front foot is anchored in the right spot, I can move my back foot wherever I like. Being mobile makes a massive difference to my comfort and toeside upwind angles
On other foils it's a bit harder as I don't have the 37cm rule. I believe (correct me please) that moving the mast forward (closer to the front foot) increases the weight on that foot, moving it away decreases it.