Basement slabs are never completely level, as anyone setting large tiles on one is sure to find out. I used over 10 bags of Jiff-Set on my most recent basement tile job, just filling the dips as best I could. But the tile really needed a perfectly flat surface, as the edges had no bevel at all (cut square) and the grout lines were only 1/16". I did the best I could with what I had, but still ended up with a lot of edges sticking up. So the question is, if you really need to get the basement slab perfectly flat, how would you accomplish it?