What's your height and what size board did you buy? Did you setup the board to the reference stance width?
Generally reference stance width is a good starting point unless you're very tall or short relative to your weight, or if you bought a board that doesn't match your weight.
You said that on the floor 15/-5 was most comfortable but you then said you only tried 20/*... So why not try 15/-6 or so? That's actually a great starting stance for most noobs.
I'm 6'1" 185lbs and got a 159 burton process flying V.
I am at reference width now. I put it at what is comfortable when i'm just standing but as soon as I strap in, it feels weird on my right knee again. My right foot just wants to lift up on the right side of the foot...it just doesn't feel natural having my foot completely flat at that angle. however, i can't find an angle where flat is working for me.