ResistanceMatrix[g_, prec_:$MachinePrecision]:= With[{m = PseudoInverse[N[KirchhoffMatrix[g], prec]]},
Outer[Plus, Diagonal[m], Diagonal[m]] - m - Transpose[m]
]
rm = ResistanceMatrix[GridGraph[{10, 10}], 40];
rm[[12, 68]]