Ok, then you need to decide whether you will do your computations from the skin surface, or assume the current is at the center of the conductor. A skin calculation will be quite abit more difficult. To be honest, the whole idea of trying to accurately compute these things from formulas, for the real world is mind-boggling, if not futile.

I'll bet they use some sort of supercomputer simulation, to get formulas with fudge factor constants for different conditions and materials.