I am trying to get an orthographic render out of Blender with precise accuracy. The scale that I would like is 1"=1'. To prepare my scene, I have changed units to Imperial with separate values. I then changed the output dimensions to 7200x5400 - so that I can get an 24"x18" output at 300ppi. My problem now is setting the orthographic scale. I have tried 12 (for 12 inches in a foot), but the resulting render is too small. After fiddling with the scale manually, I figure that I'd need something less than 7.4, but I don't know how to get a precise scale for my use case.
Unfortunately the answer by Duarte Farrajota Ramos does not address the orthographic scaling in imperial units. However, after pulling my hair out for a few hours, I finally understood that the Blender cameras do not respect the scene units (is this a bug?). So what I did was to set the orthographic scaling to 0.9144 so that it would match a yard (it seems that a yard is a better target than a foot) and then adjust the unit scaling in the scene properties, and voila, it works!