Believe it or not we set it so "it looks good" ... yeah it's not science but we use the coilovers to achieve the best stance for the car. Keep in mind this could mean a different setting on all four corners to balance it out.
Here is a pic of a car we setup this weekend (look at the fender to tire gaps for reference):
We never have to touch the rear height, seems Reg and crew always set it well. The front is a piece of cake to adjust. Simply jack the front end up and spin the lower spring stop up or down depending on which direction you need to go. Measure the change on one side and repeat it on the other (the amount you spin the stop up or down) since they maybe starting and ending in different spots.
For reference in this picture:
Spring is bright red, seat is light grey just below it, and just below that is dark grey stop you are adjusting.
When you let the car down you need to make sure the spring seats properly in the upper mount/seat too since it can come loose when you fully jack it and take the compression off.
Just want to add that the example photo here is a car with no engine or transmission so it is really cranked down to simulate ride stance for photos.