Chimney height is important, but a properly built fire box is critical. Chimney height is a much easier fix. If your fire box is not built correctly, it could turn into a major problem. A smoke shelf is designed to allow a down draft on the backside of the flue and draws air in on the front through your damper after being stopped by the smoke shelf. If your fire box was built correctly, you should not be able to see the light of day when you look up your chimney. This is why I went through 5 masons when choosing one to brick my house. One actually told me he would stagger the flues on an angle all the way up to slow down a down draft.