Second, the plunger in the middle doesn't thread down to the hub, instead it's held in place by a spring mechanism and drops at the press of a button to one side of its head. It will therefore end up in precisely the same spot each time, on the face of the hub, with smooth action and no threaded inconsistencies.
Third, the main feet have stand-off feet attached, which can slide up to the indicators for 29”, 27.5” and 26” wheels, with the capacity to check the dish on wheels from 20-29” and all hub combinations. So, by lining them up correctly on the scale, you can ensure the stand-off feet will sit on the rim, while the fixed feet clear the tyre. Tyre size will therefore have no effect on the result, so that you'll be able to judge if and where spokes need to be tightened with accuracy, without removing the tyre.
Designed as a surprise birthday present for Dave Rome of CyclingTips, the Harbor Gauge Wheel Dishing Tool is anodised a beautiful green and is simple and quick to use, giving extremely accurate readings. You can have confidence that every wheel you ship or service will be perfectly on dish.