It's a combination of springs and pre-load spacers. Get the springs you think you will use first, then add pre-load spacers.
Put in the springs only then drop the car from about a foot above the ground. If you want "bones level" at the back, add spacers and repeat the process until the bones are level when the car settles.
Make sure it's at race weight first, i.e. motor, radio gear, battery all installed.
If you decide to change springs after a shake-down run,go through the whole process again, as it will settle at different height depending on spring tension.