The height (H) of a sharp V 60 thread is 0.866 times the pitch. The points forming the roots should be cut off @ 1/8H. Some British threads are cut with a radius in the roots, as well as on the crests to fit. Probably not that much stronger than the flats, but sure enough a pill to cut.
The Tap drill size for most threads gives approximately a 75% thread. A very good approximation of this, if it is not a super critical thread, is to subtract one pitch length from the threads nominal OD. Realize that English threads are given in threads per inch so one pitch length is equal to1/tpi so tap drill for a -20 = - 1/20 or .200". On your Handy-Dandy little pocket calculator just enter the 20 & hit 1/X.
