As stated, the only time to use a number of Sq. Ft. per AP is in the earliest stages of estimation. For that I use numbers that range from 2500 up to about 3500 Sq. Ft. per AP and this is totally dependent on a number of factors. Construction material, client density, ACI/CCI, what type of traffic patterns we are planning on seeing, what applications are planned in the near term etc. All of these factors as well as running the AP's at 1/2 power so that in the event of an AP interruption, the surrounding AP's have available power to increase their power to fill in the coverage gap that is created. It is never recommended, from my years at doing RF planning, to run AP's at full power as ARM will move the AP's power and channel up and down based on the surrounding environment.
Usually after doing a rough calculation I then do an RF Plan, either using Visual RF in Airwave or using the standalone VRF PLanning tool available on our support site. From there if it is a challenging RF environment I will set up an AP in Backup/Persistent mode and go out to the location and spot check, that what I thought I was going to get for signal, is what I am really seeing.
Hope this helps.