Manual says to use drill bits as gauge but numbers?

^^^this^^^buy a tap and bit set and you'll get a #25 bit for a 10-24 tap now go to your local tool store and try and replace the #25 .

PS you will notice that neither chart gives you an actual size that you can walk in and ask for as a replacement #25 but that 10-24 tap is one that I use a thousand times a year doing door hardware .

You gotta be kidding me. Any tool store that does not stock common number-sized drill bits is not a real "tool store".