I agree Tim, referencing the edges/faces is much more important than finding the center lines. When I snap lines for laying out a wonky square rule piece, or for scribing, I level and feather mark the timber, then plumb my top line down on either end, and then put a level line either 1.5" or 2" down on either end and connect the lines. Now I have marks delineating two planes at 90 degrees to one another.