If you want Room Reserve to calculate sales tax, simply log into ALL BRANCHES mode (ignore this step if you have a single branch library).

Click on System Configuration and Settings, System Settings.

Set the tax rate and click save.
The system will automatically add that percentage on to the reservations and label it as tax.

Note: this feature is available on updated versions of Room Reserve. If your system does not have a tax feature, contact Evanced Support to get on the latest version.