Look through the posts and see how others have leveled their ribs with aluminum angle prior to laying their floor. If your floor is a little springy between the ribs, you can glue-screw some strips cut and placed long ways (bow to stern) between the ribs to stiffen up the 1/2 inch ply. Three to four 2" strips placed this way will really give the ply some backbone. The strips will go on the bottom out of sight. Oh, I guess I should read before responding. Just 12" between ribs. No prob.