Decide on Exchange Mail Server

We have around 3 mail servers (the exchange server name you would put in the MS-Outlook client) for each of the two locations where our mail servers are . The basic idea is to segregate users based on their location.

Each of these mail servers has around 16 different stores where the mailboxes are actually created.

When a new mailbox has to be created, someone has to manually go through the store reports to determine the number of mailboxes and the space used in each of these stores.

Is there a better way of managing this? Can someone give a set of rules that can be used to manage the stores, so that none becomes too overloaded?