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Anything that can magnify the measurment locations and can provide a reference marker on the lens will work. A dial indicator on the movement bed will provide the most affordable way to measure extremely accurately. If you have a cheap high power loupe laying around that should also work. Pull out a hair and put it across the lens. Rifle scopes use Black Widow Spider Web for the crosshairs. Since you are using this for a linear measurement and not trying to locate a center, the eyepiece can be located in an offset position from the spindle. As long as you don’t bump it it should work.