The key to unlocking this map, as Simon Miles so eloquently discussed and monumentally discovered is this - the English inch!
I discussed with Simon whether this was in fact why Boudet laboured the point about the English Language - he wanted his 'mystery' to be discovered using the English inch.
I'm sure he won't mind me reproducing part of the Rennes Celtique map with the grid added [above].
As de Sede wrote;
'No one except Boudet calls the Roche Tremblantes [the Trembling Rocks] the Roulers. Let us look up what Ruler means in English..'
Look what de Sede has intimated. That the word Boudet uses, Roulers, phonetically sounds like the English word Ruler ... Ruler in English means ...... something to measure the length ... rulers are marked with standard distance in centimetre's and inches.
From this we can discover much more about the Cromlech!