Calculate G's of an Accelerometer
If you use a 10 bit a/d converter to digitise a voltage in the range 0 to 5V then each count will represent 5/1024 = 4.88mV

The sensor data sheet gives an output of 8mV per g acceleration.

In the case that you are stating then the offset from the zero reading is 30 counts.  This equates to 30*4.88 = 146.5

at 8mV per g this is 18.3g

From a system level I would have a number of concerns (and these really depend upon the end application for which the device is being used)


+/-250g is one hell of an acceleration.  This sort of acceleration can only be seen for a very short time.  In order to capture this you would have to have a fast A/D converter, but then the sensor has a 400Hz filter on the output which will remove the fast transients from the output.
The resolution of the A/D only allows the determination of the acceleration to steps of 0.61g.  Is this resolution sufficient.
Any noise on the A/D input can give a 1 or 2 count error in the A/D conversion resulting in a large error in the measured value.


If you scale/amplify the output from the accelerometer and limit its value to to lie in the A/D converter input range then you can, at the cost of a reduced dynamic range, improve the sensitivity of yor device without having to increase the resolution and cost of your converter.