handy,
Divide your 400,000 BTU by 12,000 for tonnage figure.
It would be shortsided to only add 25,000 for expansion and growth.
If you are in a responsible financial institution, you also have to consider the issue of "redundancy". You might have four separate AC units in a computer room whereas three would do to meet the tonnage requirements. This is so you can have one in reserve for breakdowns and/or be able to shutdown one for servicing.
Just some thoughts.