It depends on whether you run the lengthwise grain from front to back or side to side on the cushions. it also depends on which way your pattern runs in the fabric, and whether you intend to add separate side pieces or not. Generally any kind of pattern or nap to the fabric will require more fabric, and if your pattern is "railroaded" (i.e. runs on the cross grain, rather than the lengthwise grain), you will require a bit more still.
I just had all the cushions in my Trillium
5500 redone, and went with the most efficient fabric layout possible, with a fairly solid looking fabric -- the cushions all zip at the back, with the pattern running from front to back -- no seams in the front, and gussets sewn at the corners to create a nice box shape. Took about 10 meters all told.