In looking to find the bottom center of the hull, I've stumbled on a nice shop tip. Located a small 1 pound hand barbell with round end pieces and started it rolling until it stopped. That is bottom center.
I use a level across the gunnels, when level, find the center on a thwart, and then plumb boy down to the hull and mark, do the same at the other end and snap a line in between the 2 marks!!
It would not even need a barbell. Once the boat was proven level even a short piece of pipe would work. Or a ball if you stopped it rolling towards center.
Except on the Hogbacked Saint, where the pipe would probably roll around like one of those Get the Bee Bees in the Clowns Eyes and Mouth toys.