Heating calculation is done by a heating calculation booklet called J calculation, which is now in software. It takes into account what the walls are made of, how thick, how much insulation, how many windows, what type, e glass, 2 pane, wood, vinyl, location lat, longat. sun exposure, cubic footage of each room, how many rooms, size stove in kitchen, how many bedrooms, IE people give off heat. This can be purchased online, fill out about 3 to 4 pages of infor and its plugged into software. The calculations of heat load are sent back and many calculate the size of ducts for each room. I did some intern work at GE and use to do the paper booklet, since no one wanted to do the math and reference charts. RJF
https://www.loadcalc.net/
Software sample RJF
https://www.elitesoft.com/web/hvacr/elite_rhvacw_info.html