I'm new to all this so please bare with me 
The distance for the arm is roughly 4-5 inches. The solenoid should be independent of the arm. It's main job is to push a button on say a cell phone to unlock it. So if power stays on then that will turn the phone off which is bad. So a brief on and off state for the solenoid. The arm with the stylus will land on (since were using a cell phone for example) the screen and slide the "unlock" bar down or across. So the distance is small.