Marc:
This is how I would do it:
1. Set
Special Snap Points to Half (default setting for me).
2. Hover the cursor near the mid-point of the Wall to get the Special Snap Point at the middle of the Wall, set the cursor to that point (check mark icon), issue
Drag command, click on this Special Snap Point of Wall.
3. Issue the
Snap Point Constraint command and let the Snap Guide intersect the perpendicular walls, and click on these two points to set the mid-point of the Wall halfway between the two perpendicular walls.
One of those operations that looks complicated when written out but is simple and fast once the commands are learned (and the corresponding keyboard shortcuts are set).
David