My experience is from choosing springs for coilovers.
Once you get to C/O springs you have a finite number of spring lengths. Usually you end up with a choice of two lengths due to the shock length. After you select a spring rate, the max compressed height of the springs usually determines the spring free length.
If you go to afco racing they have two different spring lengths for a 5.5" diameter srping. if you go with too long and two soft of a spring the compressed height will be too long and the spring will bottom out before max compression.