Reply With QuotePublished torque limits are generally based upon the failure of the minimum thread diameter of the screw under the combined stresses of tension and torque that the screw experiences during its installation. As far as the strength of the mating part is concerned, the failure of this item is based upon the shearing of the internal thread of an equal strength material of appropriate thread engagement. As a simple, and quick, determination of the required minimum thread length of a matching part of the same material I have generally just looked up the thickness of the standard nut of the same strength classification as the screw/bolt.
A general determination of the required internal thread engagement of an alternative metal material can be made using the ratio of the shear strength of the new material relative to that of the standard nut; and this should work for machine screws since they are generally not forged in the manner of high strength bolting nuts. You notice that I have focused upon the internal thread length ratio rather than a torque ratio, this is because it is generally assumed that the strength of the mating internal thread will always be designed to take advantage of the full strength of the screw.
Accepting all of the variables you have listed that go into a published screw torque, , for metals, ie aluminum, the adjusted torque could be based upon the same ratio basis given above; but, for your composite materials, that is not necessarily the case because these materials probably do not have the same shearing modes and characteristics as metals.
Long story, finally short, depending upon the criticality and safety requirements of your application screw joints' design, I would suggest that you are better served by using your own current test method for determining the appropriate torques for the composite materials at the thread engagement lengths that you desire/require for your application(s) as long as the reduced screw grip load at the lower torque is acceptable.
