Igaging Calibration

[RCWorks]

I just installed an Igaging DRO, the EZ view... While working on a new product I was making a mount for an X axis stepper motor. To make the first unit I used an edge finder to measure the length of the stock using the DRO. Then I divide the number in half and go to the center. I use a transfer punch to mark the center as a reference since the mounting hols for this plate require it to be centered. I drill the 1.5-inch for the stepper register hole. then I move up to drill mounting holes. I go back to -1.500 from 0 then drill the next at +1.500... I look at the part and it looks off. The part is 3.800" wide. I check the 1.500" register and it's short of center by 0.020 that the last mounting hole s short 0.400. This error is consistent and my slide was set with a depth mic from the surface of the mill table. It's withing .001 over 16 inches.

Any way to calibrate these units? I really don't want to by another 3 axis DRO.