Get it close, then do it by feel. I put bindings on a new board a few weeks ago, mounted them exactly like they came off of my old board. On the first run, first heel turn, I knew they were wrong. Leaning on a heel edge, I couldn't get over as much as usual without a LOT of effort, so I knew I needed more heel overhang.
In other words, if it feels like one side of the board is HARD to get up on edge, move your bindings closer to that edge.