I went from 24/12 to 18/-9 and my back leg kept wanting to move further back. I finally settled on 15/-12 this season but I've been recently waking up in the middle of the night with knee pain. I learned to ride in the early 90's and have the problem of opening up my body to face downhill instead of aligning myself with the board so get a lot of pressure on the inside of my back knee.
Back to 15/-9 I guess, hopefully it won't feel awkward. My point: keep experimenting and always listen to snowolf!