Greg Robidoux keeps track of available space:
Keeping track of your transaction log usage is key to understanding how your database transaction logs are being used as well as keeping track of how much space the transaction logs are using. Depending on the recovery model that is set within your database and also the backup jobs that you run, the size of your transaction log can vary quite a bit. So what is the best way to keep track of the transaction log usage?
Read on to see how. This is almost assuredly a part of any monitoring system but if you’re working to roll your own or find yourself in an environment without such a system, it’s a good thing to track.