Maybe you want to define the walls from the inside out, so one of the corners of the 'phone hole' is at a 0,0,0 in your coordinate system.

Then, if Jolla's new phone is a different size, or the printer you're using is inaccurate by a given amount, you can apply a scaling factor to the whole thing (in any dimension) without causing the phone hole to move away from the origin, making aligning other parts with it or redesigning things easier (you can use simpler calculations for offsets, and you can still snap to a grid in the software)

?