What is the correct method to determine the drill bit size from the measured threaded diameter?
When you look up a chart of drill sizes, you want clearance sizes. As an example, this drill chart has the drill size used to create holes that will be threaded with a tap, as well as the larger size you would use to create a hole that will clear the given threaded fastener. It actually gives two clearance sizes for that. You might want to use the larger if those two sizes for non-precision work that has more than one fastener, otherwise you could end up with alignment issues between pieces, where the holes don't line up.

Here is a wikipedia table of the same thing.