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.

Thanks Ray. You guys have been more than helpful in my quest to go fast.This has been a thorn in my side for a while now.Any and all other advice or feed back on my setup would be greatly appriciated.