A voice command is not instant, as the speech recognition engine waits for you to finish speaking, after which it must process the data to attempt to match what you've said to a possible phrase, which also takes time.
If this control requires a fast response, which it likely does if the overshoot is that significant, you'll either have to use a non-voice trigger for the stop command, or use preset hold times(E.G. you'd say "increase by 10 percent", and the key would be held down for the appropriate amount of time, meaning you estimate the percentage increase required from the current value to the desired value before issuing the command).
The latter assumes that the response to the control is linear(I.E. the increase from 0% to 10% takes as long as that from 10% to 20%, and so on).
One other option to consider, which is not usable with all controls, is to always reduce the control to 0 as a known starting point, so you can set an absolute value(I.E. always reduce by 100%, so no matter the initial position it will return to 0%, then increase to the desired setting).