Your bench watts will be higher than your air watts but it is a reference number. If you wattmeter will fit in the plane and will maintain max watts, then you will see what your take-off watts are. I would expect this to be the highest number.

RPMs are done with a tachometer which is an RPM meter.

Assuming you have not over propped the motor, you should be able to estiamte based on KV and Volts sustained by the pack on nder load.

If you are running 11 V and this is a 1000 KV motor you should be turning 11,000 RPM on the motor. Assuming there is no gearbox, then that is what the prop should turn.

On the other hand if this is a 550 KV motor, then you will get 5500 RPMs

Based on RPMs and the pitch of the prop you can calculate MPH, assuming no slippage.

11000 X 6" = 66000 Inches per minute/12 = 5500 feet per minute or a little over 60 mph pitch speed. I think 45-60 mph would be a reasonable top end target for this plane.

at 5500 RPM X 6" - half of that or 30 MPH. I think that 30 mph might be a bit slow as a top speed for this plane, but with a large enough prop, it might have a terrific climb, or do well as a tow plane for a glider.

That's how we play with props.