Your right, but when you've charted for years and every month is 30 days long , I think it's pretty safe to say its a 30 dAy cycle. Luteal phase is usually assume to be 14 days before next period, although, like you said, that too can vary, but if someone who has consistent 30 day cycles, if they have a "normal" 14 day Luteal cycle, they will ovulate 2 days after someone with a 28 day cycle, hence changing the due date.