distance between points

Hey guys, 

I'm having an issue with creating a line in a file I am working on.  I have the feeling it's got to do with data management.

I would appreciate if someone has a moment to take a look at my file. I hope the diagram file is easy to understand.  

I'm working on creating glass panels in between mullions, choosing the outer edge of the mullions and extruding this line the height of the distance in between an "upper" and "lower" mullion, for a total of three panels per block(in the GH file, upper, middle and lower).  I want to link the height/ extrusion lenght of each panel as the distance between mullion and mullion.  

The problem arose when working with the distance between A and B points, since it would measure the distance in the red line, instead of the blue line (diagram).  I worked it out by shifting the list, but ended up with the lowest window not showing (because the entire list was shifted by one), and having to manually create this window block later (in the GH definition, Lower Window)

Could someone explain me how to properly measure the A-B distance in the blue  line without having to shift the list and therefore loosing the lowest window block?  

Thank you very much!

David