The taller stacks will draft a lot harder than a shorter stack. If your pit drafts too hard it can cause several issues, constantly having to add wood to the fire box and lack of smoke flavor are the 2 most common problems. You can build your stack a little taller than the calculator recommends but I would put a damper on it just in case you need to slow the draft a little. I like mine just tall enough to keep the smoke out of my eyes but not too tall to fit through the garage door.
Remember, it's based on internal volume of the chimney. You can increase or decrease the diameter of the pipe to get the desired height while maintaining the required volume.