I am practicing my first serve and I have no idea how fast the ball is. So my usual reference is try to have the ball bounce once inside the box and then it should hit the fence behind. Anybody can guesstimate a minimal speed for that to happen? The ball should at least hit the fence with some speed, perhaps enough to make it "stick" on the fence itself. The size of the court is just a regular public kind, may be 15-20 ft between the base line and the fence?