data sheet page 3:
Sensor Input Measurement range (typical) +/- 3.6g
Sensitivity (typical) 300 mV/g
Zero g Bias Level (typical) 1.5 V
These tell me that an output of 1.5 V means no acceleration
and that the acceleration is measured at 0.3 V/g, so 1.8 V is 1 g in + direction, probably up from the chip's point of view,
and 1.2 V is 1 g in - direction, probably down from the ships point of view.
Page 5 pin functions and function descriptions has a drawing of the chip, with x, y, and z axis shown in the center of the drawing,
thereby defining the + and - directions in 3 space.
500 ug * 0.3 V/g = 150 uV