For a given airfoil, with known Cl and Cd curves, how can I calculate the autorotation AoA range?
This NACA report may help you further.

Basically, determine $v_i$ = the induced velocity in the hover, then vertical autorotation speed is around:


2 * $v_i$ for a rotor with 12 degree blade twist.
1.8 * $v_i$ for an untwisted rotor.


More detailed information in Prouty chapter 2, although the annoying non-metric constants pop up all the time.