Have you verified that gravity is working correctly where you are?

0g should correspond to half the supply voltage.

So, assuming a 3.3V supply, 3.3 / 2 = 1.65V

If 5V is full scale, 1.65 / 5 * 1023 = 337.

So, 340 and 369 are not too far away for x and y, assuming the device is roughly horizontal.

Now, somewhere on the device's datasheet there is a figure for sensitivity, expressed in "mV / g" probably.

Apply that to your z (gravity, acting downwards) value, and you have your conversion.