The norm these days is 12* -6*
The old way 30* 15* was for carving. I still use this and can beat ANYONE down the hill fakie. (taking bets)
But these days, the masters of the terrain park definitely use the duck stance of 12-15* and negative 3-6* on the back foot. Extra wide 24" or so. I advise of measuring from the top of of you knee to the bottom of you foot for a freestyle stance width.