I just make a rough room shape or wall first. Then I drag the corners and on screen it shows you the inner wall dimensions as you do this. If you zoom in enough it can be as precise as 1/8” accuracy. I’m not sure what the metric system precision is if you use that. The drag precision works with room floor shapes and dimensions too. I never use the wall modify properties except to alter the wall thickness or height that I need. I don’t use that to set a length.