I have never lived in a house with an AC that was "too big" but have
been in many with an AC that was too small. Some of them have to run
nearly continuously to struggle to maintain a decent temp. Whatever a
sensible design program comes up with I would round up another half
ton unless I was really really sure it's answer was good enough.

I thought conventional wisdom was that, if it's running "nearly
continuously", and not "continuously", it's big enough.
The only time an AC unit or heater is too small is if it's
running constantly, and STILL can't keep up, or if you're
after a faster recovery time from setbacks.