The first thing you need is a CONSISTENT, REPEATABLE ANCHOR POINT. (That may anyplace from your upper row of teeth to just under your eye AND from rather forward on your face to quite a ways back. There is NO "one correct place" to anchor.)
THEN, and only then, take a long arrow and draw it to your anchor, while an assistant clearly marks where the arrow meets the back (i.e. the far side) of the bow.
Do that several times to check the consistency of your draw.
Unless you have a severely cramped shooting style, you're likely to be approaching a 30" draw.
Mark