Garbage in, garbage out --- the futility of false accuracy. There are too many unknown and unknowable factors here for much real precision. For example --- us

Garbage in, garbage out --- the futility of false accuracy. There are too many unknown and unknowable factors here for much real precision. For example --- user environment, expectations and individual tolerance. How effective is the insulation? Was it installed properly? Are doors and windows constantly being opened/closed? Do the primary occupants expect a constant temperature? If so, what is their preferred temperature? How much variation is acceptable? There can be a large load difference based on the answer. And of course, any provided answers are subject to change with the occupants. Bottom line --- anything you do is really just a best guess. All this leaves two practical options, neither of them really good 1) risk that an undersized system will still be acceptable or 2) oversize the system and risk that extra operating costs due to any decreased efficieny will be acceptable. Most HVAC people that I know tend to lean toward option 2 because the only way to fix option 1 is to rip it out and try again --- and owner's tend to have a very low tolerance for the large expense involved.