Going farther

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For minor leveling problems, floor leveling compounds or thinsets can be used. The amount of correction possible is limited by the strength of the fill materials. Most floors can be adequately leveled using this compound before applying the finish floor or cement board underlayment. If using Hardibacker or Wonderboard underlayment, the thinset is applied to the subfloor, and the board is laid into the thinset while wet and secured with nails or screws. Keep a 4' level handy and the floor will be level. You will need a toilet flange extension to raise the toilet flange to the new elevation for any filler, backer board and tile.

The ultimate fix to a very uneven floor that has a sag or dip(assuming no underlying structural issues)is to cut out the underlayment and subfloor and sister lumber onto the existing joists. This need not be full dimension joists, (2x6 is fine) just lumber screwed into the side of existing joists. It is set flush to the high spots, and raised high enough so that the low spots or dips are brought to level. Each Joist must be made level, and each parallel joist must be level with the ones adjacent. When new plywood or OSB subfloor is applied the room will be perfectly level. This is quite a bit of work, and I have only did it once, 20+ years ago as part of a major renovation, and never expect to do it again. As far as I know it held up fine. This is not a conventional approach and others may comment or have opinions on this method. I would be glad to hear from them.