OK did that test.
Im moving forward against the dial indicator in X in steps of .010'', these are halved on the dial indicator as in DIA mode.
I moved 5 steps of .010'' forward and 5 back, so from 0 to .050'' and back to 0 again.
Here are the measurements for each tap od .010''
forward movement of .010'' moved .0045'' on indicator, so .0005'' short of .0050'
forward movement of .010'' (.020'') moved .0095'' on indicator, so .0005'' short of .010''
forward movement of .010'' (.030'') moved .0140'' on indicator, so .001'' short of .015'
forward movement of .010'' (.040'') moved .0195'' on indicator, so .0005'' short of .020''
forward movement of .010'' (.050'') moved .0241'' on indicator, so .0009'' short of .0250''
I then reversed the taps to move X away from the dial indicator
backward .010'' (.040''), dial reads .0200'', spot on half.
Backward .010'' (.030''), dial reads .0149'', so .0001'' short of half .0150''
backwards .010'' (.020''), dial reads .0102'', so .0002'' over half .010''
backwards .010'' (.010''), dial reads .0050, so spot on half again.
backwards .010'' (return to zero), dial reads 0.