OK, it took a few reads, but I think I understand. You want the left side of the highlighted wall to be aligned with the left side of the short outer wall below it. The walls are different thicknesses. Because the walls are positioned based on the center line of the wall, it is tricky to make them align.

You have two choices.

1) Zoom in to max magnification and nudge the inner wall left and right with the arrow keys until they are visually aligned on the left. (Shift+arrow jumps a greater amount).

2) Do some math.