Have you downloaded our free program, LINKPlanner?
If you use LINKPlanner to create and configure a link, you can generate an installation report that provides the expected RSL (dBm) and voltage at the BNC connector.
The lower the voltage, the better.
Please see this forum post for specific RSL voltage conversion: http://community.cambiumnetworks.com/t5/PTP-820-Licensed-Microwave/How-do-I-use-the-PTP-820-BNC-connector-RSL-interface-during/m-p/40337#M78
I hope this helps!
Dave