so my slab was poured all the same height for living room and bathroom. the living room floor gets 3/4" and 3/4" (1 1/2") on top of the slab to finish. I am putting tile in the bathroom and heated matt under the tile. what should I do to make up the 1 1/2"? the tile and thinset and mat might make 1/2 to 5/8- should I put a thick grout base in then the matt and tile or do I put 1/2 tile board down then go.....