The numerical value you read on the analog pin is a linear function of the voltage on the pin, and should be 0 at 0 volts, more or less.

I believe your device's voltage is a linear function of Gs. 0g is 2.5V (at 5V supplies), and it will vary some number of millivolts/G. The ADXL202E is 312mV/G, but I don't have a datasheet for your part handy. Look it up. You are looking for the "Sensitivity" section for values expressed in mV/G.

Near the end of the datasheet it has a handy note on calibration. Turn your part so gravity gives you a +1G, measure. Turn your part so gravity gives you a -1G, measure. Calculate constants for your part. Isn't it handy that you are supplied with a 1G sample planet to make the math easy?