illustrator: setting stroke width in mm instead of points

Also untick Scale Strokes & Effects (Preferences > General, or in the Transform panel menu) if you're resizing things afterwards. Otherwise you set 0.5 mm and it quietly turns into 0.37 after a scale.

Preferences > Units, change Stroke from Points to Millimeters. (On Mac it's under Illustrator > Settings.) You can also just type 0.5 mm into the stroke box and it converts it for you.