You have over 2,000 posts and still don't have your location in your profile?? Do you think that might be a factor here......maybe
The very general rule of thumb is:
Total heat=Area X CFM X Change in temperature.
Lets assume a constant CFM. Total heat = 900 X 30 = 27000/12000 = 2.25 Tons.
Which is pretty close to the usual 1 ton per 500sq/ft
Assuming 8' ceilings, less than 20% window area on the South side, That you use drapes and have awnings, The walls and ceiling are adequately insulated. The list goes on.
Do you want a high energy rating?
Is sound rating important to you?
danny