There has been a misconception about pneumatics for teams that don't regularly use them. I would like to stress,
pneumatics and hydraulics are NOT the same thing. Hydraulics and pneumatics both operate with fluid flow, however the main difference is the type of the fluid, the pneumatics operate with air, therefor a gas; while hydraulics operate with a liquid, it is very difficult to compress a liquid, gas on the other hand will fill their container( just one of the properties of gasses).
If you're still interested in trying to achieve position control with a pneumatic cylinder take a look through
THIS thread.
(sorry for spelling out the differences between hydraulics and pneumatics, It's just important to understand.)
One other option, (I don't know if variable pressure regulators are legal in pneumatic systems) try using a variable pressure regulator, you may be able to slowly increase/decrease the pressure in the system and measure the current height of your lift mechanism with a sensor and adjust via sensor input. Just an idea.