Speaking of measuring, I eventually realized that this eyeballing method was hopeless, and I bought a caliper. It's not designed for this kind of setup, though it did an adequate job of indicating the distance from the concrete to the bottom of the board.
Not leaning on the board, marker in place
Carry the caliper to the board to be shortened, line up the end of the caliper with the end of the board, mark where the "upper" jaw hit the bottom of the board, make a mark.
Extend the mark so I can see it under the saw.
