Thank you @airscapes for your very insightful post. I had extrusion width set to auto. Changing it to manual produces walls that are almost spot-on 1,20-1,21 thick
Filament thickness and how much it deviates through the spool. You must measure and set actual average thinness in the slicer
tests were run on a quality filament
Extrusion multiplier and how you have it calibrated it. I have found using a single wall square will end up being under extruded.
For that purpose I have always used the abovementioned cube: 25mm cube, 0% infill no top, no bottom, 3 shells for walls. Used average thickness of all 4 walls as a base for extrusion multiplier calculations.
Extrusion width, are you using manual or auto (I prefer manual)
that was it!
Perimeter over lap percentage
I have it at 40% (it's the Outline Overlap, right?)
starting Z offset to a small degree
I have it dialed in pretty well