Backlash comp is there to take up 0.0001" to 0.002" of slop in the mechanics, ie leadscrew and nut, ballscrews. bearing preload, belt stretch, motor coupler tolerances. these things do not vary over the course of axis travel, Any mechanical mechanism has to have some varying degree of tolerance, "backlash" if it didn't, it would be welded tight and not move. If you have different amounts of backlash over different portions of axis travel, then you have have different problems, that can only be corrected by replacing the defective component. generally your leadscrew or ballscrew, or worst case scenario, your ways, or linear rails are wore out.