I read all the time that your contractor should perform a heat/cooling load calc, and if not you should find someone else who will, but over the past few years I've had several contractors out for a couple of different projects (including installing central air for the first time), and NOONE is crunching those numbers. I even asked this last time and received the "I've done 800,000 of these.......". But when the units come in increments of 1/2 tones, what's the practicality of such a calculation anyway? Or is more than sizing the unit, like number of supplies/returns, where they are located, etc.?