Formula For Calculating Length of Overlay For Welding Broken Earths In S/S

Please can you help with the formula for calculating the length of the overlay of the materials for welding broken earths in substation. I have information on the type of material (e.g copper) with resistivity of 1.72 X Exp.-8, melting point 1084ºC and required c.s.a. mm² / kA of 12.5

My question is if I have to weld (using brazing method) two similar and dissimilar materials together what must be the overlay size in terms of length and breadth?

Thanks