In my shop (24 X 30) I ran 5 rows side to side. Each row is 20 feet long, and the rows are 6 feet apart. The rows are made up of 2, 8 foot-4 tube T-8 fixtures and a 4 foot, 2 tube T-8.
With this setup, I don't have any dark areas in the shop. Unless you count under a car, then I put a 4 foot single tube T-8 on the floor, under the car, problem solved!