Using an ARI manual J. This allows you to see the needs as to the direction of the doors and windows. It shows different load needs for a different orientation of the home. The calculations are the same for the north as for the southern states with one exception. The sun states should be calculated using 105F degrees as the ambient temperature. The manual is straightforward and easy to use if time is spent reading the directions. Consider it an investment in future energy savings. Any salesman that guesses should be run off the job. The process only takes about 1 hour for a novice who read the instructions and even less for someone that regularly does the calculations. Using blueprints can make the calculations very accurate.