Hi, I have a regency I2400 insert and am going to sweep it myself as it is designed to be swept from in the house. The stove manual says the flue is 6", so I assume that the SS flex liner is 6". Question: Do I buy a round a 6" brush or a 7" brush? I am looking on www.fireplaceessentials.com and they have the round poly brushes. I just need to know what size to use in a 6" liner? Does the 7" press better against the sides and clean better? Thanks in advance.