I assume you have no torque specification listed on your plans or in your instructions? Certainly my plans and instructions have nothing on this torque value but then mine are Ye Olde Original from back in the "Black and White days" (as my teenage daughter likes to tell me). Additionally, back in my day (hahaha!) the electric flaps were optional and they came with a separate sketch on how to install them - ie: they were not identified on the aircraft construction drawings. This may or may not be the case for you as it would depend upon the age of your kit.
In the absence of a specification from the manufacturer of the equipment it is normal practice to use standard torque values for the thread size you're dealing with. You can go online and find these values listed in a number of places - just search for something like "standard torque value for UNF threads".
If that gets too hard then I would use the very sound advice provided by Walt.
On my electric flap system I drilled the corner of one flat and lockwired the nut to the actuator. You could probably also use a locktab if the rod-end bearing has a groove for one. The use of locktite is another excellent idea on this part.
