As a general rule once a plant begins flowering it will get 25-30% taller. An 18 inch plant should finish 24-28 inches or so. If your unsure use 50% as your guide, once you know your strain you can dial it in better. Sounds like you have a little room yet. Why is your light so far away? I have a grower using LED'S and I swear his are 8-10 inches max. I don't use them so just wondering...good luck