Actually Grahame it is a simple formula and I wasn't being patronising. I apologise if that's the way it came across. I agree that testers may be able to measure resistance pretty accurately but the whole point of my lengthy posts have been to push the point that not all testers may be able to measure, i.e. resolve, the differences in resistance due to heating, poor contacts, thermal emfs, etc, repeatably and with acceptable accuracy, at lower test currents in Copper earths - That was my point. Isn't the ability to observe small changes in resistance (not noise), whilst manipulating a cable, what we should be looking for if we can't drive 25A through a cable to test its integrity?
