So to get this clear: you're talking about a key press on a keyboard connected to a computer, which then in turn sends the value as ASCII character over Serial to the Arduino? Because this means you're basically looking to have the Arduino react to a command received on the Serial bus, and how that command is generated is irrelevant to the Arduino.
This in contrast to a keyboard (or button) connected to the Arduino itself, where the Arduino does the keyboard reading.
Then there's the turning of the monitor itself. That sounds like a rather heavy thing to move, so you have to keep inertia in mind when controlling the servo. Asking it to move as fast as it can to the new position may be a very bad idea: the monitor is moving at it's fastest by the time the new setpoint is reached, and then still has to slow down.