If i'm casually standing on the floor with shoes, it seems I need about 15 degrees on the front and -5 on the back.
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.