Hi Aldo, to measure any spring they use a combination of weight and distance... On a luger spring it would be measured as to how much weight it takes to make it compress (deflect) On most compression springs, usually one inch, and also all the way compressed which is called "solid" ... then they measure how far it returns to free length, the difference between new free length, and used free length is called set, and will continue to set, degrade, over use/time...... Almost all original 9mm springs are 21 coils. almost all .30 are varied, 18 or 19 coils.....best to all, til...lat'r...GT....
BTW, no matter if you trim a coil or two.. the spring still has basically the same working rate, all you really change is the preload?... Which on a Luger is very close to the working design range, (spring doesn't move much!) so it might affect closing more so than opening??.....