Assuming you already have rectangle contraints – horizontal, vertical, width, height, etc. – then I’d probably constrain the centers to be coincident.

Yeah, you can see all those constraints in the picture.

Constrain one with height and width equal to parameters, and constrain the other based on 80% of the dimensions.

Ah, okay - this is more or less the way that I usually do it. If its a box, the concentric separation, or “wall thickness” would be the same, so I would set a vertical and horizontal distance constraint between the outer and inner rectangles. This just felt sort of hacky to me, so I was wondering if there was a dedicated constraint so that I could, for example, select the outer rectangle, and the inner rectangle, then click a “Constrain concentric” option. Then I would only need one constraint for the wall thickness.