Displacement is the basis for comparison between fleets in Jan Glete's book, Navies and Nations. It turns out that Jan Glete is using the navigational draft for computing block coefficient and displacement. I can understand that it might be "fair game" to define a method and that would be a useful way of camparison. One issue is that you would not be able to compare his displacements and block coefficients with other sources, where they are computed using different standards.
Both Ab Hoving and Frank Fox arrive at much greater block coefficients than Jan Glete. The reason is largely that the more standard block uses the mean draft, rather than the navigational draft (the maximum draft). Jan Glete's block is larger for the same displacement, so the block coefficient is smaller.
Block coefficient = (displacement x 35 cubic feet/ton) / (water line x beam x draft)