The only place I've ever seen that kind of statistic used is when a datacenter is leasing space by the foot, and leave it up to the customer to figure out how to allocate power. The method for figuring out how to convert this into something actually useful for the likes of us:
- Take the total square-footage, multiply by 250W to get total power for the room.
- Take the measurements of the racks you plan on using, and get their footprint (probably in Sq/In since that's easier for us USians).
- Figure out the needed clearances fore and aft for servicing, add to the footprint of the rack.
- Convert that sq/in number to sq/ft, multiply by 250 watts to get your wattage per rack. To convert to amps, divide by voltage of choice.
- Look at your cage-layout and figure out what parts of the floor aren't actually in the above (door swing areas, aisles, the spot you're storing the ladder, that kind of thing) and compute the percentage of the total floor area consumed by live-system rather than these dead-spaces.
- Take the square footage of live space and divide the total-power number by the life-space number.
- Rerun the computation in step 4 to get your actual power-per-rack number.
A lot of math, and will take some diagramming to figure out, but you can get there.
It's a pain, though.