Interpretation of
"Spreading Resistance of Circular Source on Circular Disk with Edge Cooling" may well hold the answers.
Think a deeper understanding(than I have ) is needed :
"Spreading Resistance of Circular Source on Circular Disk with Edge Cooling" (when applying zero Edge Coeff ) appears to give different results than
Isotropic Disk with Convective Cooling .I would have expected to be the same(maybe silly me but.....)