To Keep It Simple: You want to match the middle of the tapered plug (shown in red in the graphic) to the ID (inside diameter) of your threaded hole.
Example: The ID of the leading threads of a 1/4-20 hole is 0.196". So, you need a tapered plug where the center of the plug is approximately the same diameter.
Tapered plugs are the top choice of most powder coaters for masking threaded blind holes as they're easy to install and their contoured shape can allow them to fit multiple hole sizes.
There are 3 dimensions: Top Diameter (A), Bottom Diameter (B), and Plug Height (C). In order to find the middle diameter, you just need to add the Top Diameter + Bottom Diameter and divide by 2.
Middle Diameter of Tapered Plug = (A+B) / 2