for the backup battery (the units are millivolts, IIRC).
As for the main part of your question:
While this is in theory possible, there’s a better way: using a control loop, like a PID. These were very popular last year in order to regulate flywheel speed. There are about a billion different implementations of PIDs in ROBOTC (even one implemented directly) , you should be able to find one to fit your needs.