id put your back at -12. alot of it is preferance though i have my bindings angled at 21 and -18 but thats just what works best for me onw thing you could do is loosen your bindings then adjust your feet to a comfortable position then check the angles and tighten it down.
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.