I am still a fan of watts per square foot. For me I take the smaller of the following two:

1. The total electrical capacity for IT gear (critical load)
2. The total cooling capacity at the rack (not CRAC capacity)

Then divide this number by the square footage of the raised floor space. I find IT people like watts per rack (because they just try to fit stuff in the rack) and facilities people prefer watts/sf (because they have to power and cool the stuff).

To convert, an average data centers require 25 sf per rack (includes aisle ways and CRACs). The range I normally see is 20-30.

I also find that when that simple conversion math is done.... IT and Faciliies are 10X off. "What??? We can't do 10kw per rack with air cooling?" (400w/sft)

Every data center is so different the "standards" usually become "guides".

My two cents.