Disk loading..
Is just a static number, run on a calculator. a 7.4" diameter prop will have 43 in^sq. Don't worry about it too much, as long as you have a known airframe weight. For example, the DDust weighed 56oz/43 in^2 = 1.3 0z/in^2, so I guess 1.3oz/in^2 is pretty vertical. I don't have any published numbers, but I never saw anybody who thought this was underpowered
Run as many RPM's on the ground as you can, with as small diameter as possible, and as much pitch as your engine will pull.. it really is simple..if you run a lot of un-needed diameter, you can't turn any pitch. No pitch=no speed. So lose the 11" diameter props.., lose the 6" pitch..get you a prop that turns about 8~9" of pitch at about 1K~1.5K less than the maximum rated HP RPM of your engine..it will unload to right into the max rpm range of your engine..then you will be going as fast as your engine will allow..
Oh yeah..you need to set the pipe to run at this unloaded RPM...and some prop/plane/engine applications will unload more than 1~1.5K..the vrdf was probably unloading 2~2.5K, so my pipe was a little shorter than calculated, to anticipate the unloaded RPM's..
It's all just numbers..and fuel..and props..You'll get it pretty fast, or know that it ain't working.