Creating custom scale measurement for QGIS project

I started to make a map based on a raster (a scanned paper map). The map obviously has not geographic coordinates or a projection.

The map comes with its own scale, and I would like to somehow make my QGIS project use the same scale.

As you can see, the map uses a scale of 1 inch = 5 linear arpents (the arpent is a French area unit which also has a linear counterpart). 1 linear arpent = 58.471308 meters.

If I use the measurement tool, it says that the inch measures 0.016 meters. How can I set QGIS to accept that from now on, this value shall be 5 arpents ? I would want my scale box in the printed form of my map to be accurate.