B
Bill
A string line across the levels would have worked too - that is how we used to lay falls in floors.You put a piece of the packaging card board that the tiles come in under a tile placed at the highest part of the floor, stand on it and that gives you the finish height of your highest tile, do the same with a tile on the lowest part of the floor in all directions and then with either a straight edge or a chalk line with a screw drilled into the floor at say the mid point if it's a large span, use these as height references to set your straight edge on and then when you lay a tile make sure that it has a solid bed and that the 'tilt' of the tile sits flush with the straight edge, follow this across the floor and then you will arrive at the highest tile it's at the exact height and plane as the highest tile on the floor. It took me a while to master this technique and it's good, but it's not as good as proper prep!
In this instance I also had an external door frame to consider when planning how to deal with this particular problem, that's why when all things were considered I chose to 'ramp' the floor down over what was a large space
Nice job.