The best way to understand how well insulated your house is, is to do an energy audit with a blower door on the house and vacuum load on the duct work. This audit is also all telling as far as what is required to make a building less leaky. Duct work is a huge problem as it is not sealed or insulated properly in many homes. The use of long runs of flexible duct work also causes great inefficiency in the system. Giving up your conditioned air to the crawl space or the attic - not a good use of energy! Also makes for a very uncomfortable house.
Using computer based heat load analysis tools is the proper way to determine load. The standard that we use is Manual J. This information is used to properly size the new equipment. Many times (most) the new equipment is half the size of the existing equipment. Upgrading windows in a house can make the existing equipment oversized. Oversized equipment short cycles on milder days and is very energy inefficient.