Ed,
42.0 , 42.0 = 0deg (obviously)
42.0 , 43.5 = 1deg (43.5 is 1.5mm more than 42.0)
42.0 , 44.2 = 1.5deg (44.2 is 0.7mm more than 43.5)
Hinge pins moving through arcs would make it less linear than that but at such small angles that can probably be ignored.
It's basically saying that every 0.75mm is worth about half a degree.