[Mark McLeod]

The numbers -1 to 1 only refer to the battery voltage as it is at the moment.
It won't give you a specific RPM and will vary as you syphon power off to other motors (like your drivetrain), and as your battery gradually depletes itself during a match.

To get a consistent RPM you will need a feedback sensor and what's called a PID software loop to maintain a set RPM.
For feedback you can use an encoder (some come in the Kit of Parts), or a light sensor that counts the passing of a marker on your spinning shooter wheel, or various other types of measuring sensors/devices.