You compress the spring full in a vice and measure it's coilbound length.
In my experience, my yellow contra bound up before full variation, don't know if the red is the same. It was about 2mm to shave off the spring to let the pulley travel all the way
Btw, might sound silly but if you need to make a short sleeve to keep a larger spring centered internally on a driven pulley, the outer race from a standard moped crank bearing works excellently welded to a washer