It's only meant to be a rough guide to compare props of different pitch and diameter. It just compares the swept volume of the prop per revolution.
To give an indication of a comparison between the static thrusts at the same rpmyou'd multiply that number by the pitch (Force = rate of change of momentum)
To compare powers you'd then multiply THAT number by the achieved rpm
But it's all a bit hand wavy, because unless you're armed with the known speed of the model and handbooks of characteristic curves for the props, you're going to have to experiment a bit anyway.
I've just discovered one of the downsides to having larger models; the cost of the props. I've had a 16x6, a 16x8 and now an 18x 5 on my Edge540. and it's still not quite right.