Linking quantities

Is there a way to link two or more quantities together? One example would be, when working on conceptual estimates, I do a gross square footage of a building. I use that quantity for other quantities (i.e.: electrical cost per sf, rough carpentry, etc.). The way I do it now, is to either copy and paste the item and rename it, or duplicate as, then rename. It would be nice to have a quantity, then if that quantity changes, all the other linked quantities also change automatically. i am doing this manually.