Using Surface Insulation Resistance (SIR) parameter to calculate resistance between pads

what is the correct equation to utilize a SIR parameter to calculate the resistance between two SMT pads on a printed circuit board?  Is the SIR reported by a IPC B-24 test coupon the same or must it be adjusted for the geometry of the board to reflect the correct unit-less specification?
So far what I see on-line is:  R=Ps*(G/P) where Ps=SIR, G=gap between the pads, and P=the perimeter of the space between the pads.  is this correct?

Thanks all!

SIR has little relation to the resistance between two SMD pads at standard temperature and humidity and typical board voltages. It is the measure of the breakdown characteristics of the board materials under high humidity and temperature levels where electrochemical processes may lead to breakdown under much lower voltages than with a clean, properly manufactured board. The leading cause of these breakdowns are from salts or other contaminates that are introduced in the flux / soldering / cleaning process.

I appreciate your comment, but from what the IPC says, not entirely correct: " electrical resistance of an insulating material between a pair of
contacts, conductors, or grounding devices that is determined under
specified environmental and electrical conditions".  I want to take that value and derive the resistance between two pads at 2V.