How to calculate aerodynamic efficiency at minimum power conditions?
At cruise condition, a propeller propulsor can be modeled as: $P=TV$, with constant output power as a function of airspeed. Your objective is to find the cruise condition such that the output power is minimized.

Assuming you know apriori what altitude you want to cruise, then density $\rho$ is determined. At level flight:

$$T=\frac{P}{V}=(C_{D_0}+KC_L^2)\frac{1}{2}\rho V^2S$$

At cruise, weight is equal to lift. At minimum power, we have $\frac{\partial P}{\partial V}=0$:

$$\frac{\partial P}{\partial V}=\frac{3}{2}C_{D_0}\rho V^2S-\frac{KW^2}{\frac{1}{2}\rho SV^2}=0$$

Do some algebra, and we have:

$$3C_{D_0}=KC_L^2=C_{D_i}$$

Therefore, the minimum power condition is not equal to maximum L/D (as an aside, it is equivalent to the condition for maximum rate of climb, which is pretty intuitive). 

Now you can find the corresponding $C_L$ and verify whether it's within the valid range of the lift curve. Generally, we want at least 1.3G margin to stall warning or stall buffet. So a 1.4G margin to $C_{L_{max}}$ might not be a bad place to start.