How far up S**t Creek am I?

Flat is important but level is not as far as tile is concerned. Try to get some
idea how much and where material needs to be added/removed then decide the best
tactic. Your reference book should spell out reasonable allowable thinset
thicknesses. You may want to rent a concrete grinder if there are areas that
need it to get the thinset to the right thickness. It helps to use a long
straight edge to find high and low spots, like an angle iron about the length
of the room. Survey leveling compounds as another way to fix the problem. If
you're working with a good tile suppliers, they can help a lot. Good luck.

Joe