Been doing some digging on good old google and playing with lines on the map. Bordeaux is south east of cherbourg. It’s a direct line of site from cherbourg to angers to Bordeaux, and the have a cathedral where the only original surviving structure is the nave. They protected it and rebuilt the cathedral around it.
Contrary to that nef was translated to nave as a literal meaning. But nef is also a paper fold meaning a ship. It originate from ancient French which he has been using all the way through this clue. May be a port directly south of cherbourg as this would cross the water twice (neptune’s help).
Starting out from Cherbourg I followed the general direction from the picture (SW) looking for an opening and a heavenly light. I paid special attention to fixed landmarks at quite a small scale on google maps and it didn’t take long to find such a place. The problem I had was that I’d Crossed the water once so far and a second journey was going to take me far across the Atlantic. In fact in a straight line I landed in northern Spain. Initially I thought this just wouldn’t do at all, as I already had it in my head that all marks would be in France. To my relief the village I landed in was connected sufficiently with France for me to mark the location. From here it made sense to make the turn and head straight through the opening on my map, through Bourges..onwards in search of the becalmed nave. For verification I was satisfied that my landing point tied in with the 600 nave and only just passed through my opening created from the 580.