Coilover Rear Adjustment question
 

So a good friend of mine explained to me how he adjusted the rear suspension on an OEM style coilover setup. My question is, what is TRULY the proper way to adjust the rears? Such as how do you decide the strut length to the spring height? I searched a lot and never really found anything about how to adjust it. I have my coilovers in now but obviously I wanna be sure I set it right. Thanks in advance.