Im building a stand for my 240 using Rockets plan with 2x6 rails and 3/4 plywood top and bottom. The top will also have a leveling mat from the manufacturer. My cement floor isn’t level. Looks like there’s between .25 and .50 difference from left to right. What’s the max that can safely be shimmed? Are composite shims better than wood? I wouldn’t think they could, but can adjustable feet be used?