There is a spacer on my threading dial which is essentially a stand off. It allows the dial to be rotated in or out of engagement with the lead screw. The thickness is not critical and timing is not a concern. It depends on how much room is available on the mounting screw.
The black spacer is barely visible off the lefthand corner of the dial body.
You can adjust the length of the spacer so that the marks line up correctly when the half nuts are engaged. Or, you can loosen the dial and rotate it to accomplish the same thing. You want the half nuts to properly engage when the lines line up, not in between. If they are off, use either methods to correct. To test, engage the half nuts and push the apron back towards the tail stock to take the slop out. Adjust until the lines line up on the dial.