How do you control the length a piston will extention/retraction?

[MrForbes]

You cannot accurately control the length a piston will extend, if there is a load on it, because the air in the cylinder acts like a spring. You're better off using a motor instead of pneumatics if you need to have good control over how far something moves.

The neat thing about this year's game is that for the most part there is no need to have good control over how far something moves...I mean, the game piece is 40" across, and it just has to go over or under the overpass, it doesn't have to be any specific height. We're using pneumatics that are either up or down for our 3 ball manipulator mechanisms.