Ditto on what fastsky posted. I probably should have been more clear. You must take a baseline measurement on your engine/prop. For example, your engine can turn a 10x6 APC 2 blade at 10K RPM. Punch this into Case 1. Thrust HP claims a arbitrary load of 60,000 and HP of 0.429. To find another prop that will load the engine similar at that RPM, on Case 2, put in 10K RPM and then fiddle with diameter, pitch and # of blades until the load figure closely matches Case 1. The program isn't perfect as there are too many varibles it doesn't take into account, such as engine power curves. For example, a 6x6 APC 2B at 20,000 RPM shows the same HP as a 10x6 2B at 10,000 RPM. Odds are, a TT46 will never get anywhere close to 20K RPM, so use some common sense when using the program and it will get you in the ballpark. Cheers.