Indeed, the difficult lies in the fact wall's length is measured by its middle and not its exterior

No the biggest difficulty comes from the program can' t guess if you want to draw a concave room. As soon a room is concave, your idea doesn't work.

You know, you're not the first user to think about this issue. There are many threads speaking about it in this forum. At this time, the easiest solution remains to draw the room with the room creation tool then double click in the room to create the walls around it with the wall creation tool. Of course, this works well if you have only one room to draw.