Not sure where you get xlength x correction factor?
Although Ze should = Zs + (R1+R2) the R2 value often has parallel paths so does not always work out in practice. I have seen lower values for Zs than Ze especially on a TT system.
Ze should be measured without any other earths connected and since we are allowed to get our Ze by enquiry maybe the equiry method is best.
There is no requirement for the supply authority to supply your premises from the same transformer at all times so if you measure the Ze today it may be 0.25Ω and tomorrow it may be 0.32Ω so it is better to calculate using the 0.35Ω TN-C-S enquiry figure.
Or in the case of a TN-S supply 0.8Ω
In this way if the supply is changed you are still within the limits.
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