The formula FeatureCAM uses is from the Machinery's Handbook Ed. 20 Pg. 1435:
drill size = outside dia * 1.299 * thread_percent * (1/threads_per_inch)
drill_size is the size of the drill that will be used to rough the hole.
outside_dia is the size of the tapped hole -- for a "1/4-20", it would be 1/4in.
thread_percent is Thread % for tap drill from the tool selection page of default attributes. By default it is 77%.
threads_per_inch is just that. in the "1/4-20" example, it is 20.
The drill size table in Machinery's Handbook is computed based upon 75% thread percent (pg. 1442 of the 20th ed.),. FeatureCAM's default is 77%. Switch to 75% if you want the same behavior as the handbook
Also, it should be noted that FeatureCAM's tool selection tolerance is 0.002 inches. This tight tolerance occasionally results in the failure to automatically select a tool. For example for a 5/8” hole, the formula results in a number that is very close to a standard drill size given in the handbook's table, but not quite within 0.002. Hence tool selection fails. In this case you will have to explicitly select a tool.