[scottj]

elmariachi describes exactly, the method I used to determine the shock length that my car required. The only thing I would add is that there is a ride height that is correct for your chassis and it is determined by the suspension arm angles. When I adjust the coilovers I use a digital level on the arms as well as a tape measure. Because of the shape of the E-M body, it is unlikely that you can achieve the low-in-the-rear look and have the rear suspension angles within spec. I ended up having E-M lay up a modified body that is lower and re-shaped in the rear.