When I poured my slab, I just dug a 3'x3' area 8" deep where my posts were going, very little extra concrete and provides some mass in that area for the concrete to cure a bit more stable, should help with controlling sag. Keep on the finish guys when they are pouring to make sure they do a good job tamping that area to keep it stiff and so they don't work in any low spots. The effort here will be worth it when you are install your lift and are trying to level it. Looks like a fun project !!
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