My general rough rule and first setting on a new server is to set it so sql server max memory is 75% of the total available memory. Then see how it goes.

My servers are generally small (2 to 16GB) and that's fine until server team (who don't understand sql server) go altering the available memory on the VMs and of course never tell me.

They keep trying to take memory away from sql servers and, because their vSphere 5 monitoring tools suggest it from a one off snapshot, they want to run our test Sharepoint server on 608 Mb of RAM and 1 virtual CPU and reduce our (already slow especially for reporting) 2 instances development & test sql server from 2cpu to 1 and from 2Gb memory to 1Gb. Grr!