Ball screw lead

What am I missing? I realize the advantage of ball screws is
the "zero" back lash advantage, (much better than the lead screw in
my cross slide), however how do you get the incremental movement.

On my lathe the cross slide has a .002 per increment and a 40
increment dial. This means that the distance in one revolution
is .080. Most of the ball screws I have found THK for example
have .500 to .200 lead per revolution. Seems like there is a loss
in incremental movement.

Please help a beginner.
Thanks and Regards
James McGuiness