Moved this code here from _validate, as it is really part of setup, and not validation.
The original used a redundant instance variable (self.coord_decreasing), not used anywhere else.
While fixing that, also replaced the calc with something equivalent that seemed clearer to me
 -- note that the original was working hard to only change exactly what it needs to but I reckon any efficiency benefit is trivial as there are only ever "a few" interpolation coords.