Isn't it amazing how expensive the fabric becomes once you start figuring out how much you'll need?! In my case, I determined how I was going to build the panels and then estimated how much fabric would be required to cover what needed to be covered. I found a fabric I liked and it was conveniently 60" wide. It made a huge difference to have that extra width. In some cases it allowed me to use a single yard per 2x4' panel (depending on how I made them). In other cases, I still needed 2/3s of a yard to cover the back and one full yard to cover the front.
I guess if I were to generalize, I'd say 2 yard per 2x4' panel is probably a good rule of thumb.
Composer, researcher, educator