If there is no bedrock, the lower boundary of the compressible stratum is calculated at the central point of the foundation slab. You can invoke the Compressible Stratum Calculation Point command from the Edit menu and indicate a different location of the point with the cursor (it will be indicated by a red cross).
If the shape of a foundation slab is close to a rectangle, it is recommended to place the compressible stratum calculation point in the middle of a line that connects one of the vertices of the rectangle with the intersection point of its diagonal.