There"s a Rule of Thumb that works pretty darn well for US Standard (aka American National), Unified, AND ISO Metric threadforms: Calculate the tap-drill diameter by subtracting the Pitch from the Major Diameter.
For US Standard and Unified screwthreads, the Pitch is equal to 1 divided by the Number of Threads Per Inch. So, for a 1/4-28, the Pitch is 1/28 inch.
For ISO Metric screwthreads, the Pitch is used in the size citation . . . M6 x 1 has a Pitch of 1 millimeter, a M8 x 1.25 has a Pitch of 1.25 millimeter, and so on.
The Major Diameter is essentially the outside diameter of the male thread. So, for a 1/4-28 the Major Diameter is 1/4 inch, which is 0.250 inch.
The thusly-calculated Tap Drill Size is 1/4 inch - 1/28 inch, which is 0.250 inch - 0.036 inch = 0.214 inch when expressed in three-place decimals.
My handy-dandy chart says the 1/4-28 Tap Drill is a #3, which is 0.213 inch.
The difference between 0.213 inch and 0.214 inch will, in hardware reality, be within the hole-to-hole size variation of holes drilled with the smaller drill.