In reply to "Points" are meaningless.:
By my zeroth approximation (aka wild-ass guess), you'll need a number that is the app's em divided with the summed fractions of ascender and descender.
Sample for a made-up combo of app and font: 1200/(650+150)
where random_app's em is random_font's em of 1000 + its leading of 200, the tallest ascender is 650 dits above the baseline and the deepest descender reaches 150 dits below it.
Btw, 'dit' is very likely not an accepted technical term.