Precise inset?

Add a plane make it say 4cm x 3 cm. Now I want to inset the exactly 3 mm. How do I do that? Or what is the best way to achieve this.

I use a mirror modifier and move on side over 3mm then the top down 3mm then apply mirror mod. I thk there is an easier way to do all this.

I’ve tried inset but it won’t let me do metric.

Inset (shortcut I) with a thickness of 0.003

Wrong thread.