To be exact, the pitch dia.= major dia. - 1/2(major dia-minor dia.). Having said that, there are tolerances for min. and max values for major and minor diameter of threads, therefore, there will be min and max values for the pitch dia.
Pitch dia.(max;min)=(1/2)( major dia.(max;min)-minor dia.(min;max). I just derived this last equation but you may want to carry it out with real numbers and see how it compares w/ your Machinist's Handbook. a
Ok, then how do you calculate the minumum minor diameter? So that I may apply it to your formula. The minumum minor diameter is not listed in the handbook.