Right; I said the same thing. If the shroud length is constant; the amount of stretch required is the same. But you can't apply this universally to all shrouds because different boats have different lengths of rigging wire. If you plug in 4000 mm for your base length; the delta L will double to get the proper tension. Since that's true you can't use the 1mm/5% rule to get exact tension. If I tighten an intermediate 1/4" shroud using that rule the tension will likely be ~40-50% of breaking load because the shroud lenth is much shorter than the upper shroud; which goes from the masthead to the deck.
As I understand it, if you mark off 2m of a shroud 6m long and then tension it to 10% of its breaking strength you should find that the marks that you made are now 2.002m apart.
If you mark off another 2m of a shroud that is 10m long and you tension this shroud to the same amount your 2 marks will again measure in at 2.002m apart.
this sound logical to me but hey...I am no expert, I could be wrong.