Ok - since the 2 ends have *different* numbers of splines, if you leave one end where it is, and just go down ONE notch, then you'll lower that side around around 30mm.
If, however, you REMOVE the torsion bar completely and move the trailing arm to the exact position you want, you will find that you can slot the torsion bar back in at *some* point of it's 360 degree rotation. What I mean is, the relative position of the splines at either end will offer sufficient adjustment to lower the car by any amount. It's this fact that lets you choose the height.
Andy