Section the hole either the long or short way on the csink and dimension the width there -or-
My favorite method, one that is not used enough, is to dimension to the theoretical sharp of the cone of the countersink, which is, regardless of the shape of the upper surface, where the theoretical sharp of the cone on the mating screw is going to be. The best part is that it directly controls the depth the tool is pushed into the metal when the csink is made, unlike the diameter, which is affected by the cone angle and the surrounding surface.
Either way, use a section and it will be clear to everyone.