Find Set distance between two angles

I have a series of angled surfaces grouped together in two’s. At each group of two surfaces I have 2-3 location.

I am trying to identify at which point the distance between the surface is 6 1/2" horizontally while being centered on the joint. I am not really sure how to go about this at all. Any advice is appreciated (11.7 KB)
Angles.3dm (353.6 KB)