I usually just build the legs taller than needed and make sure all my cross beams are level and then trim the tops off the legs before I add plywood or whatever
I filled 4 tanks to see what would happen. The shelf, which is supposed to handle 2000 pounds sagged about an inch and a half. That was 675 lbs. Far from the claimed 2000lbs they claim it can hold. I shimmed up the shelves so I suspect that it is a moot point now.
