Hello,
Walls are measured in their middle as you noticed and this can’t be changed. This subject has been discussed many times in the past: as there’s no way to guess how users will draw the next walls, there are many situations where measuring the length of wall side wouldn’t be reliable.
So either you take into account the thickness of the walls when you draw them, or you draw a room first with the Room creation tool, then add walls around it with a double click in it with the Wall creation tool.