I'm trying to draw up some precise plans in millimeters and get caught in a bit of a resizing cycle sometimes.

It goes like this

- Draw a circle for a screw hole that needs to be precisely 2.40mm
- Adjust stroke width to 0.01mm
- Screw hole width/heights change slightly to 2.43mm (or whatever, just an example)
- Fix screw width/height back to 2.40mm
- Stroke width changes to 0.008 mm
- Fix stroke width back to 0.01mm
- Usually ends here but occasionally I think it's changed the width/height yet again at this point

I'm often tweaking stroke widths etc so I'm finding this incredibly frustrating to work with.

Am I doing something wrong? Is there any easier way to do this?

with the selected you will see in the control bar above the canvas some (possibly very tiny) icons right after the units drop down. The first of these, if you hover over it will pop up with a description "when scaling objects, scale the stroke width by the same proportion." for your purpose this option should be deactivated. Now you can resize without having to set your stroke width back each time you make a change