Why Thread Engagement Isn't 100%
A tap drill is deliberately undersized relative to the thread's major diameter, leaving room for the tap to cut full-form threads without binding. Most general-purpose tapping targets around 75% thread engagement, which gives nearly full thread strength while keeping cutting torque and tap breakage risk reasonable.
When to Drop Below 75%
Hard, gummy, or stainless materials chew up taps fast at high engagement percentages, so many shops drop to 50-65% engagement in those materials to cut breakage rates, accepting a small reduction in thread strength in exchange for a tap that survives the job. Soft, free-machining materials can often run at or above 75% without issue.
Rounding to a Real Drill
The calculated diameter is rarely a stock drill size, so round to the nearest drill in your index, slightly undersized rather than oversized if you have to choose, since a slightly tighter fit is easier to fix than a thread that's too loose to hold load.