Here's what Culver Propellers says (using a backer plate) :
Torque Specifications for Propeller Bolts:
1/4” bolts . . . . . . . . . . . 132 inch pounds (+ or – 10 inch pounds)
5/16” bolts . . . . . . . . . . . 144 inch pounds (+ or – 15 inch pounds)
3/8” bolts . . . . . . . . . . . 200 inch pounds (+ or – 20 inch pounds)
7/16” bolts . . . . . . . . . . . 250 inch pounds (+ or – 25 inch pounds)
1/2” bolts . . . . . . . . . . . 300 inch pounds (+ or – 25 inch pounds)