These fireplace guidelines are provided as a general sizing chart for fireplace relining. They were calculated using a 10:1 ratio.
If your chimney exceeds 25' it is recommended you use a 12:1 ratio.This ratio is the square inches of the opening of the fireplace compared to the square inches of the liner.
If your fireplace opening is:
Example:Your chimney measures 20x30 or 600 square inches. You will need a 9" liner to hold the volume.
Your first paragraph ...