My terra cotta flue is 13"x13", descending to a 6" hole in my hearth, connecting to 6" stove pipe into my Sierra Hearthstove. Based on the 6" hole and 6" stovepipe, I ordered a 6" flex liner kit from Rockford, 30' long to reach 2 floors up to the top of my chimney plus an extra 5' or so just in case. Insulation kit too. Does this sound right? The reason for this seemingly idiotic question is, the sweep's estimate calls for 40' of liner. Can't figure out why.