After looking at one of the CC rib racks out on the cooker, I think average sized half slabs would do just fine, and I think using all six slots to cook three slabs would be as well. I was just outside looking at it, and all six half slabs would still be pretty much over the pan. For evenness, I'd recommend having one set of tongs in each hand and doing two rib swaps per rack. Simply swap each outer half slab with one of the inside slabs midcook and leave the 2nd and 5th slots alone. Of course, you can also simply go by how they seem to be cooking.