The "old school; shop-rat" rule-of-thumb for tap drilling is:
Diameter - 1/pitch. i.e. 1/4-20 gives .250 minus 1/20 (.050) = .200
If you use the lookup chart it will give a #7 drill (@ .201")
This gives very closely 75% thread depth; a very goodbalance between tapping effort, & thread strength.
(works even easier in the metric system!)
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