An easier way is to clock system run time on a design day. Do so after thermostat has been at the same set point for at least 1 hour. In most houses you will find the furnace runs less than 50% (aka 2X the size needed).
My old furnace was an 88k, never ran more than 5 minutes before hitting setpoint unless it was recovering from setback. New 44k furnace runs about 20 minutes out of 30 at 17f design conditions.
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