I like the 7 x 4 still. The RPM numbers are similar to a TD 09 as I run them with a Cox 7 x 3.5. You need a lap time around 4.5 sec. Slower is better, but small airplanes need to truck along to fly well. I'd guess 55 ft lines will be about right with the 7 x 4. From experience, not calculated. I think you will find that the higher pitch props will load and slow down in maneuvers. Anyway, it will be interesting to fly and try and see how actual flight relates to your calculations.

Are you building the Rascal from plans or the German kit?