how smooth is smooth??
 

I have a room that was 50% tile and 50% carpet on top of a concrete slab. I pulled up the carpet, tile, and as much of the thinset as i could. There are spots where there is 1/8" thinset still there. I'm not sure I'll ever get that side to match the smooth carpet slab. Given that I'm slapping a pad underlayment down, is 1/8" too much?

Now that i think about it... maybe I should just pour SLC down over the whole floor and forget about the rough "tile side"

Hey
 

Well it depends, are you going to be laying carpet down or a wood floor. When I was laying carpet years ago...I almost insisted on having the floor smooth as possible...Yes that meant, 1/8 lip had to go....One way is to lay SLC..but expensive if its a large area..Another way is sanding/grinding the lip and all the thinset down..but that means more labor....either way works

Good Luck