How to have multiple configurations of a part in an assembly where each configuration is driven by an equation

I currently have an assembly where I have inserted multiple configurations of an angle steel part of different lengths. What I want to do is make one of the configurations driven by a global variable called "MAX_LENGTH". I want the next configuration to be "MAX_LENGTH" - 6in, and the next "MAX_LENGTH - 3in, and so on. This way, if I make a change in the length, I don't need to manually update every single part. The reason I want to do this in an assembly as opposed to doing it in the part file is because I have other different parts that would need to update as well (such as some flat bars). If I do the relative lengths in the part file, I'll have to update the angle.sldprt file, the flat_bar.sldprt file, and so on. Any thoughts on how to accomplish this on the assembly level? Any suggestions/comments are appreciated!

SolidworksModeling And Assemblies