The man who poured my concrete put a little crown in the center of the slab, I guess to shed water, but all it did for me was create a lot of work in trying to get the first layer of concrete block laid. I found the highest spot on the slab and used my laser leveler to shoot a laser over to the 4x4 posts surrounding the area where I was going to lay the block.
I nailed up two nice and straight boards and strung a string across the slab to give me a reference for keeping the block level. The picture below shows one of the nice and straight boards and the string and one of the little wooden doo-thingys that bricklayers use to keep the brick at the same level.
Below, you can see how the mortar is thicker at the left, getting thinner with each block to the right. I'm not a brick or block layer, so it took me a couple of hours to lay this much, and I sure wished that I had told that concrete guy to make the slab flat and level. Live and learn....
The kiln's ware bed where pots are placed inside is built on top of several levels of concrete block. The firebox is closer to the level of the ground, to the right of the block in the picture.