Thanks for the info. I found the formula on Carrier's website but was surprised the unit could be that old. An addition was added to my house in 1984, taking it from 1800 square feet (including conditioned basement) to 2500 square feet. I figured that the furnace would have been replaced then, as the A/C was.

The A/C build date tells an amusing story. The addition was built in the fall of 1984, and the A/C was manufactured in June 1985. Clearly, the owner at the time didn't want to upgrade either the furnace or the A/C when the addition was built, but once summer hit in 1985, the A/C was replaced. There must have been some miserable days before that was swapped.