OK, assume your accelerometer is a ratiometric device, with a supply voltage of 3.3V,
The 0g point will be at 1.65V.
If your Arduino has a Aref of 5V, the 0g point will read as roughly 1.65/1024 * 5 = 337.
I don't know what the sensitivity of the device is, let's say 800mV/g.
So, -1g will be 1.65-.8 = 0.85V, which your converter will see as about 174, and so +1g will be around 500.
Plug these values into "map", and off you go.