I am a couple of years late with this comment, but I understand that most of this play usually comes from a loose badly made or worn nut. Making a new brass nut is the best solution, but if lathe is old and worn you can try reversing the nut on the screw. I personally never really trust the indexers once they have been wound backwards on any cut but if you zero them and the tool to the cut using clockwise turns of the cross slide only, then move the tool off the job using the carriage only, (having never backed off the cross slide), there should be no backlash and what you apply on the indexers should be fairly correct. It often helps to put heavy hand pressure on the cross slide while it is making the new cut in case the pressure on the tool is forcing it back off the job against the screw. Lots of precision work is being done by skilled machinists on worn out equipment but they know their machines well and make allowances for the wear and tear.