As I said there are Backhills all over the place. Just for the sake of completeness, here are 117 photographs of no less than
40 (yes, forty) different places called Backhill in Moray, Banffshire and Aberdeenshire
http://www.geograph.org.uk/search.php?i=73787718. There may be others that are as yet unphotographed.
I have found, so far, three quite separate places called Fortrie.
First
http://www.geograph.org.uk/gridref/NJ6645 or, on the old map
http://maps.nls.uk/geo/explore/#zoom=16&lat=57.4985&lon=-2.5570&layers=5&b=1I can't see a Backhill of Fortrie there, but I wonder if that is perhaps another name for Backhill of Drachlaw
http://www.geograph.org.uk/photo/990519 which is only a few hundred metres (or yards) from Bridge of Fortrie. This Fortrie is in the parish of Inverkeithny.
Second, the Backhill of Fortrie you referred to above. It's a different Fortrie, and a different Backhill. Here it is
http://www.geograph.org.uk/gridref/NJ9640. It is spelled Fortree on the old maps
http://maps.nls.uk/geo/explore/#zoom=16&lat=57.4579&lon=-2.0673&layers=5&b=1 and it's in the parish of Ellon, in the real historical county of Aberdeenshire, and about 20 miles south-east of Macduff.
This is yet another Fortrie
http://maps.nls.uk/geo/explore/#zoom=16&lat=57.6204&lon=-2.4413&layers=5&b=1. This one straddles the boundary between the parishes of Gamrie and King Edward, so is probably the one referred to in that 1930 VR. Here it is on a modern map
http://www.geograph.org.uk/gridref/NJ7358. I don't see a Mill of Fortrie named there on any map, but the first edition of the six-inch map shows and labels a mill dam at the farm of Fortrie.
If your John Duncan was at Backhill in the parish of Banff, he was not at any of these other places. He could, of course, be a relative of the Duncans at Backhill of Fortrie, or of those at Mill of Fortrie, and employed away from home, but Duncan is a very common name and it could be (probably is) just a coincidence.