By the sound of it, as you are not at the level of doing major prop mods, I'd stick with a commonly avaiable prop, like a 11 X 4 APC which puts you right into rhe hunt, You probably would not notice the subtle difference. Use the pipe length from Randy's suggestion and launch RPM. In your case fine tune by moving the pipe to work best with the chosen prop and fuel, till you get comfortable modifying props. You should be good so long as the model is not too heavy.