I don't think it really matters. I just figure about...by guess and by gosh. Rule of thumb is typically off by a bit but somewhere close enough. (I actually use 10% in either direction rather than get into all the bleeding descriptions and mathematical type precision...it is just a guestimate.) Just wondering, anyway. I suspect there might be minor influances besides pot diameter and flame size. Heat production along a flame, size of center point "dead" air column, pressure of gas, actual jet spread, angle of jets, angle of wind, pot material and thichness, etc...many of which "SHOULD" cancel out by reversing stoves, but letting off pressure will cool the gas container, which effects gas pressure, which effects...well, by guess and by gosh, close enough.
Ha ha...no. Aluminum hats don't help in my case...too many security checks. They already KNOW my bad thoughts, they just want the good ones.