The most common way to design the foundation for an Isoquick slab is as a flat, concrete raft of consistent thickness. This approach simplifies the installation of the dpm and reinforcing fabric.
This thickness of the slab and the amount of reinforcement is determined by the weight of the superstructure. The heavier the building, the wider the floor span and the closer the loads are to the edge the thicker and more heavily reinforced the slab.
Using a flat slab also removes the need for edge thickenings. While sometimes unavoidable, edge thickenings complicate things. They tend to significantly reduce the amount of insulation at the edge of the slab, the very place were you need it the most. They also complicate the ground preparation and steel work required.
Typically this style of design will use between 1/2 and 2/3rds of the concrete that would have been used in strip footings for the same building, and eliminate the need for sub base walls, beam and block, separate floor insulation and screed.