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The road numbering system in France doesn't work like it does in the UK and you'd be we'll advised to mostly ignore it for navigational purposes.
Much better to use the stars and the sun. I take an astrolabe, sextant and three watches with me, along with tables to calculate the transit of Venus. It takes a while (particularly when it's cloudy) but so much more reliable and easier than following the D7 onto the N16, via St Ulf du Perdue.
