Computing it? no its not that complicated.
The pad has a cutout in the copper around with 4 bits of copper in a cross fashion from the copper to the pad, you just ensure that those 4 are of sufficient width to handle the current etc.
I.E. for a standard 1/4w resistor hole a thickness of approx 20-30 thou with a clearance of 20-30 thou should suffice, AFAIKR its not rocket science.
Just creating enough relief to allow the pad to heat up to accept the solder
without all the cooling produced by attaching it to groundplane.
Roland.