Ask around and you'll hear a version of the same rule: one ton of cooling for every five or six hundred square feet. It's fast, it's easy to quote over the phone, and in the Rio Grande Valley it is wrong often enough to be worth arguing about.
A load calculation looks at what actually drives heat into a house: how much glass you have and which direction it faces, how much insulation is in the attic, ceiling height, how tight the envelope is, how many people are usually home, and how much shade the lot gets in the afternoon. Two houses with identical square footage on the same street can land a full ton apart once you account for all of it.
Oversizing feels safe and behaves badly. A unit with more capacity than the house needs satisfies the thermostat quickly and shuts off, which sounds efficient until you remember that removing humidity takes runtime. Short cycles mean the air gets cold but the moisture stays, so the house reads 74 and still feels damp, and you end up dropping the setpoint to compensate. Meanwhile the compressor is starting and stopping all day, which is exactly the duty cycle that wears equipment out early.
Undersizing has the opposite failure: the system runs continuously through the hottest part of the afternoon and never quite catches up, and your bill reflects every minute of it.
The other half of the calculation is the ductwork. Capacity that can't be delivered doesn't count. If the return is undersized or the supply runs are crushed, a correctly sized system will still underperform, which is why we inspect the attic before quoting equipment rather than after installing it.