Earth loop resistance lowering

Sorry I obviously was not clear...
When I said " at the transformer end" I was alluding to the effect of the company spike at the base of the transformer pole (in a rural TT installation it is usually so). That neutral to Earth connection is typically guaranteed less than 21 ohms. (in your case, as your local rod will not be perfect, the company electrode must be much less, but there we go.)

So if you measure your earth rod by another method (perhaps the 3 spike method), which you would only normally do in new build when there is ne supply yet connected to do a loop test, you should allow an extra 20 ohms or so for the unknown resistance at the company end. (so if the 3 spike reading was 80 ohms, and the 'loop test' reading was 90, you might say aha, the company spike is 10 ohms..).
Of course if you already measure by loop test, all you need worry about is the 25v or 50v limit, when multiplying I trip by the rod impedance. And if you really are in the regime where 20 ohms is pass or fail, wack in another rod in parallel, one rod length or more away - some reserve is needed for dry weather.
Hope that is clearer.
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