How to get values in terms of G in STM32F3 Discovery board
You can work out the scaling of your sensor by repeating your measurement with the board flipped over. The difference between the two measurements will be 2g.
So, record your x1, y1 and z1 values. Flip the board over and record second x2, y2 and z2 values. 
Your zero points are then just the average of each pair of readings.

X0  = (x1+x2)/2.  (Same for y0 and z0)

Scaling is the distance between the two measurements divided by 2g

Counts per g =
Sqrt ((z1-z2)^2+(y1-y2)^2+(x1-x2)^2)/2

If only one measurement changes significantly (say x), while the other two have nearly the same value, then this simplifies to:

Counts per g = (X1-x2)/2