No problem at all. Check out the tool and let me know if you have questions. It is pretty powerful, especially if you decide to invest in a Datalogger, or even a wattmeter (every electric pilot's best friend). The datalogger will show you RPM, Current, Voltage and other things like temps and stuff. I have one and have built a few motor databases and it is very useful.

Remember that the 4.75x4.75 has a pitch that is about 19% higher than the 6x4, and because the diameter is 1.25 inch smaller, it can use it's power into providing thrust at speed, rather than thrust at stall. This allows the prop to propel the plane at a rate actually higher than 19% greater than the 6x4 prop. It's kind enigmatic if you think about it.

Peak efficiency is more of a guesstimate to see if we are applying the motor properly. If the motor's peak was say 90%, but we propped it for 60%, then we are probably misapplying the motor. If it is 78% and we prop for 73%, we are doing pretty well with the given motor. I think you're on to it. Keep playing around with things. I would always recommend a wattmeter or a datalogger so you can see what your actual setup is drawing to keep your ESC, motor, and battery happy.
Good luck,
C