What I ended up doing when I was in a similar situation is manually edit the XML(sh3d files are XML) with proper measurements and coordinates after first doing a rough sketch on the GUI.

I haven't used that tool for a while, but I think that it includes a table with all the items (walls, etc.) where you can enter the measurements.

Ultimately I think of sweethome3d as a sweet spot between paint and a full-blown CAD program. Nowhere as powerful as the latter but so much better than the former. Also my daughter had a moment when she preferred sweethome3d over Minecraft et al. as a building game.