OK, so I know my slab isn't level, you only need your eyes to see that. As an example; on the large sliding door I've got 20mm difference from one end to another (over 4m). I've got 5 sets of doors with level thresholds. Huge amounts of pre-planning detail was put into ensuring the doors would sit  correctly into the slab so that the slab floor was 18mm below the FFL to allow for the tiling that is running through the entire house.    I was lucky enough to be lent one of these: https:/