I believe that its 400sq. ft. per ton would I be correct?thanks in advance for any feedback!

No actually it is roughly about 800sf per ton for residential cooling.

Actually it depends upon your location and the exact circumstances of the area to be cooled.To much guesswork can on A/C size will cause cold and clammy conditions if the unit is to big and if to small it will never cool the area. To be any where accurate a manual J should be done on the structure or area to be cooled and for accurate duct work size a manual D should be done to calculate the correct trunk size and room duct feeder sizes.

HVAC equipment needs to be properly sized by using a manual J calculation. On other sights they are talking about rate increases on electric and gas. As prices continue to climb the need for proper sizing increases. Oversized equipment produce less comfort, more upfront costs, higher operating costs and more maintenance.
For more info see FAQ on heat loss/gain manual J calculations.
http://www.comfort-calc.net/faq.html