I asked Lucien about this and his response is below:

"There are several formulas out there that are close but not accurate over a wide range of props. I did a very in-depth study of propeller loads about 15 years ago, and through actual experimentation and extrapolation, I came up with the simplified formula that I use today.

You can use a D x D x P formula in a very narrow range of props for one size motor or even a group if motor sizes, but what you are doing is taking the slope of a curve, and as you get further and further away from your initial starting point, the equation gets less and less accurate.

When I used the D x D x D x P formula, I tested it on everything from a 4x2 prop all the way up to a 76x56 prop used on a Cessna 172, and it worked for every single prop."