this is for if l ride freestyle

Depends on how aggressive/fast you want to ride. Salomon riders ride way forward while freestyle/park riders are more flat.
l would go with like 20 degrees and see how that feels

if ur riding park id say 10-15 left foot and 5-10 right foot so ur feet r in a sort of duck position, this allows you to ride switch much easier and l think it gives me more balance

if u do rails or spin keep your stance wide and your frount foot should be around 12-18 and your back should be around -10

l used to work in a rental shop, we always set beginers up at 15 for the front foot, and -2 for back foot. Beyond that l think it is just what feels comfortable. My board is set at 18 front foot and -5 back foot. That is what feels right for me.