Aaah, okay, that was a very different thing, sorry....
A good question, and interesting what distance others like ?
Just checked - I ride strapless but this should give a very good measurement for a good distance, as I can put my feet where I want.
65cm is my preferred stance, and I am 175 (5'9) only but long legs.
This stance also goes for my strapped (and strapless) waveboards when in the impact zone, which is a bit wider than most I think, but I find it much better.
Stance on these are 62-65cm from center strap to center strap, and strapless sometimes even a bit wider.
If you just go straight, hydrofoiling, you can ride much more narrow, comfy - but for carving tight you want your rear foot further back (well, I do), and no downsides when cruising with a wide stance IMO.
I can see, if I stand on the floor with front foot angled, that my 65cm is where you mean 50cm from nearest points between feet ?
40cm (55 center to center right ?) sounds very very close, I would not feel good with this, apart from when cruising where it is fine.