+15/+6 sounds much better. Many many folks ride duck stance, which is -rear +front. I use 0/+12 myself. I tried -6/+12 early this season and I started getting pain on the inferior medial aspect of my rear knee. So I moved it back to 0/+12 and haven't had the pain since.
All I can say right now is that you want minor angles and, save for pain, you want to keep them where they are while you learn. When you first start, consistency is more important than fine-tune dialing (pain nonwithstanding), mostly because you're so preoccupied with not killing yourself that you really don't pay attention to what your legs are telling you about your angle.
If you've got big feet, you might benefit from moving the heelcup back, or even screwing the baseplate in slightly back towards your heelside to make room for the extra length.
I have size 13 in XL Flow NXT-AT's, and I rotated my binding disc to mount the bindings about one centimeter off center, towards the heel of my board. I actually have yet to try and mount them perfectly centered down the midline, as I'm getting the response I need the way it is.
Once you're confident up there and you're not so worried about biting it, bring a screw driver to the hill and play with your angles. Give each setup a good 3-5 runs before making a call.
And don't forget to stretch those leg muscles. It'll really help cut down on the muscle soreness. It's the first thing I do after I get out of my car at the hill.
"Never attribute to malice that which can be adequately explained by stupidity."
10/11 - 24, Smote on Feb 13