Not understanding this Dave . If I were teaching someone , the compound would be set to 29.5 degrees , same as when threading . Bring the tool in close to the piece . Engage the leadscrew to thread at a given number , dial the compound in to center the tool to the existing thread and move the cross slide in . To do this takes 30 seconds . A piece of paper and flashlight under the part will get you perfect . And I understand the compound angle doesn't mean squat . If you're not on 29.5 though , you are plunge threading and the entire tool shape will be doing the work which increases tool pressure .
I believe what you are referring to is double depth while on 30 degrees which is just an estimate old timers used .