15/-15 is the most common stance. I use it at 23 inches, but i know people that use it with up to a 25 inch stance. It took me a second to get use the the whole 50/50 angle thing, but it feels pretty normal now.
If your board is sliding out from under and you're falling on your ass a lot, get your binding disks set up so you can shift your bindings from edge to edge and move them more towards your toe edge.
I got a gnu pickle and at first the asymmetrical heel cut thing tripped me out and i kept slipping out all the time too. I shifted the bindings and it was much better.