rear foot question

Hi, all. I'm 5'8" tall and about 150 pounds (not sure that's relevant for the question I'm posing...). I'm on an 8'6" board and wondering about the proper position for my rear foot. The board has a 2+1 setup. I understand I need the foot back pretty far if I'm going to try big carving turns, rather than small, trimming adjustments, but how far back? On top of the two smaller fins? Further than that? And it seems like if I try to pop up with the foot that far back, I'll stall out and miss the wave. So, do I scoot the foot back just after the pop up? Wait until I'm about to initiate the turn? Would love any tips. Thanks!