[Aaron.Graeve]

You could figure out how a power value compares to an RPM through trial and error, but I would not recommend it at all.

Instead, if you have an encoder or a potentiometer around, you can use a PID control loop. There are numerous paper on CD of the code and use. The loop will use the present and past velocities of the arm (this is why you need a sensor) in comparison to the target velocity in order to compute an updated motor value. The motor value will get updated many times a second, helping your arm reach target velocity quite quickly ("quickly" depends on what motors, gearing, and mechanical arm you have). Good control loops represent programming on a higher level and can simplify quite a bit of work for your drivers, so they can focus on driving.

If you have any more questions, feel free to PM me.