The equation T = .2DF is a general equation to estimate applied fastener axial loading for industry standard threads – steel material.
The equation is imperfect as is the torque process..
There is more accurate force to torque equations available for critical applications however; even these equations are ultimately imperfect.
For installations that are expected to be assembled and unassembled repeatedly:
F = .75(At)(Sp)
For permanent installations”
F = .9(At)(Sp)
F = Approximate axial preload force applied
At = tensile area of fastener
Sp = proof load of the fastener
Let’s rewrite the top most equation
T = .2DF
F = cDF
F = force
D = diameter (Major)
F = Axial force applied
c= roughness approximation
Steel and zinc plated, dry threads = .2
Cadmium plated = .16
Lubricated = .16 to .17
There are equations that attempt to compensate for thread pitch and flank angle however if one needs torque and axial clamp force that accurate for industry standard fasteners they should consider alternative installation designs.
See: Bolt Torque - Axial Force Equation and Bolt Torque - Axial Force Calculator