Anytime that the drag polar can be represented by a straight line of CD vs Cl^2 (even if its just within that CL range under consideration) you will find that relationship between Vmp & Vmd (0.76 by my calcs).
You'll find some test data at the
NASA server. A good place to start, for small airplanes, is with the
Cessna 172 info here or the
info here
The question I have - why do you want to know?