The use of the transposition of the Zs = Ze + (R1+R2) formula to back calculate R1+R2 is not a valid calculation because the value of Zs obtained by measurement is a live test on a final circuit (or at least, not the origin) and so all protective measures including bonding are in place and the presence of the bonding conductors will reduce the effective value of the external loop impedance compared to the accurate measured value of Ze that is taken with the main earth disconnected from the installation.
The Zs as calculated from Ze + (R1+R2) is the worst case scenario for the measured circuit.
The measured Zs is the best (and real) case scenario and could be significantly lower than the calculated value.
Using the measured value of Zs will give an optimistically low value of R1+R2 if used in the transposed formula.
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