"Slippage" is something you could calculate, I guess, but it has no aerodynamic significance.  This paper shows how to optimize a prop for a given condition and how to calculate efficiency (and other stuff) for a given prop:

Design of optimum propellers

Adkins, Charles N., Douglas Aircraft Co., Long Beach, CA; Liebeck, Robert H., Douglas Aircraft Co., Long Beach, CA
AIAA Journal of Propulsion and Power 1994
0748-4658 vol.10 no.5 (676-682)

Martin Hepperle's Web site probably also has a prop efficiency calculator.  Maybe the best measure of prop efficiency for an electric stunt airplane is how much energy it takes for a pattern.