I just ran the liner down my chimney yesterday. I am look at about 15 or 16 feet to the top of the insert. The manual says 15 feet minimum. It's in an interior chimney. I wil not be insulating the liner. I may insulate it next year. Due to the nature of the top of my chimney, I have the option of cutting this liner in several places at the top. Is 15-16 feet enough or should I try to get it to 17-18? Will there be much difference in how it drafts with an extra foot or two.