Well, I think I used a 27 or 29/64", but it's really not that important. You want to look at where you want the bushing to set. Then use a drill smaller than that diameter. Since its tapered you can have it go in and stop within a few threads, or go all the way to the base. But if you go with the minimum size you have to ream a lot more metal out. I don't keep track I just grab a drill bit that's just larger than the tip, at the first few real threads on the tap.
The chart calls for a 37/64" (.578", or little bigger than 5/8") for 3/8" pipe and for 1/4" 7/16".
Jeff
