If yours works ok, then there is no issue. Thats the problem with thumbrules, the answer is more complicated than the simple formula. The particular combination of prop mass, prop speed, diameter of the shaft, diameter of the prop, and prop condition can all play a role in whipping.
My experience on three boats with Martec ungeared folders, with clearances from 1.0x-.25x shaft diameter, have been no problem. However, if you are setting one up, the safe answer is the minimum clearance required for water
flow through the bearing.