You need to be measured. Preferably on the bow you intend to use. We have seen a compound shooter lose 3″-4″ when changing over. Most lose at least 2″. A rough rule of thumb is this.
Take a yardstick and place it on your sternum, center of your chest. With your arms and fingertips fully extended straight out away from you take the measurement where the tips of your fingers are on the yardstick. This will be very close to your “natural” drawlength.