A rack with 4 shelves measuring 3 feet wide and each shelf being 6 inches high would need a minimum of 13.5 feet, but I would order an additonal foot to make sure you have enough. Go online and look at how the tape needs to be cut. Some of the tapes are made with the elements on an angle. So you will have to cut both ends in between the elements that are running on that angle, making sure not to cut into elements except on each edge of the tape. But that will be where you put your clips. Go online and it will explain it much better than I can. Just Google flexwatt.
Good Luck!
Jim Smith