Less than 24 volts on the thermostat C wire
Think twice before you do that, and talk to your sysadmin.  Do not do anything which would compromise the cooling in the server room.

If you don't like the balance, consider "portable air conditioner" units which dump the cool into the server room and the waste heat into the office space.  We had a big industrial unit, and it worked rather nicely.  Now they exist as consumer grade units for a few hundred dollars, so you could get 3 or 4, and only lose a little cooling if one fails.