Saw
When I am not sure I Use the .9742 ratio. It’s not dead balls on like shown on all the charts, but a few 10ths of a thou don’t bother me that much. It certainly works.
Divide .9742 by the thread pitch. Next subtract your answer from the tap diameter this will give you the tap pilot drill size.
Example. ¼-20 bolt.
Divide .9742 by the pitch 20 and you get .0487 Now subtract .0487 from the diameter of the tap .250 You get .20129 this is your drill size. Remember stainless, cast, aluminum are percentage slightly different, so apply as necessary. This will always get you in the ball park.
Hope this helps. Anthony.