Furnaces have a BTU's input rating, for simplicity lets say 100,000 BTU's and it is a 95% furnace, then we multiply the 100,000 BTU's X 0.95 and get 95,000 BTU's output. If you measure the BTU's after finding how much air the unit is moving and multiplying that by 1.08 X Temp rise between the supply and return it should come out close to what it is rated for.
