You can't
+1
It's a daft concept*, it was sometimes suggested in basic books on target archery. But it's only vaguely works for a classic target anchor.
Some anchor at the lips/chin, some the nose, some the cheek, ear, back of the jaw.
Some lean the head forward... also depends what draw weight and style you shoot. I'll draw 27" when field shooting but 30" when roving with a heavier bow.
Age comes into it too... my draw has dropped an inch in the last 15 years.
Del
*A bit like expecting to "calculate" your chest measurement from your wingspan.