Willie - the tip could be ground to 118* or 135* with uneven length cutting edges, so the rule is there to ensure the lengths are equal. Here is mine off of eBay for a few bucks.
DrillGauge.JPG (Size: 31.73 KB / Downloads: 57)
DrillGauge.JPG (Size: 31.73 KB / Downloads: 57)
Hunting American dentists since 2015.