It does depend on what voltage you use across the stick. We (and sony) use 5v through 22k to the tip of the stalk connector. This means, if the stick puts 22k from the tip to ground, the tip will be at 2.5v and you'll get 0x80 (ie, half) out of the ADC.

If you want to know the voltages, just do (outputvalue/255)*5.0. If you want to know the resistor values, do (resvalue/(resvalue+22000))*255=outputvalue

Hugo