I agree with Robert however if you are just doing a quick estimate and you believe to be with a tire kicker with the houses being built today sq x 17 will get you close enough.
However let me say as Robert mention you need the correct number so you know what your shr number is.Without them you may not pick the correct unit to do the job.
May I suggest a book from acca its the manual S residential equipment selection.This will show you how to properly size equipment.
There are rules of thumb, but they're wrong. An experienced individual can make a very acurate guess based on location, contruction, square footage, occupancy and exposure. Usually, those guesses are backed-up by load calculations before mistakes get made. Or, mistakes get made.