I did a rough check once using a largish torque wrench so I couldn't guarantee the accuracy at the low end of its range. What I did was to tighten a prop in the same way that's never given me a problem with them coming loose and then used the torque wrench. The nut started to move again at about 5 foot pounds or 60 inch pounds. This is between 1/3 and 1/2 the maximum torque that should be applied to a 1/4x28 bolt. Effectively, the crankshaft is a bolt and some are soft where others are hardened.

Tightening the nut puts the prop hub under enormous compression loads and if you overdo it then the hub could eventually fail.