What this sizes, and what it does not
This does not perform a Manual J. A real heat-loss and heat-gain calculation goes room by room: every wall, window, door and infiltration path, against a design temperature for that exact place. What it does instead is take the design figure you were already given (from a report, a design guide, or your own history on houses like this) and do the part that is only arithmetic.
Two inputs carry the answer: the conditioned floor area and the BTU/h per square foot. The rate is yours. Nothing here supplies one, because a figure that fits one region is wrong in the next, and a calculator that guesses is quoting somebody else's house back at you. Twelve thousand BTU/h is one ton, so the tonnage falls straight out of the division.
Read the result as a band rather than a target. Nominal equipment is sold in half-ton steps, so a real load almost always lands between two of them, and which way you go is a judgment about run time, humidity and the ductwork already in the house. Oversizing is not the safe direction it looks like: a unit that satisfies the thermostat in six minutes never runs long enough to pull the moisture out of the air.
The common mistake is carrying a per-square-foot figure onto a space it was never worked out for. A cathedral ceiling holds far more air than the same floor area under eight feet, which is why the cubic-foot figure sits beside the total. Write the load down against the address, because the next call to that house asks the same question. Zeus keeps job records per address with their own history, so the sizing you did once is there when you are back in that basement.
ZEUS