You just need to choose the number that gives you the smoothest cut. Normally 75 or 151 is used. The randomiser number is used in the calculation to determine the cut sequence.
I think you are confusing the Randomiser with the Control Range, it is the Control Range that determines if it is a hard cut or a soft cut. A bigger number in the control range wil make a softer cut. 200 is a typical value and for example if you have your RPM Limit set at 7000 with a control range of 200 the rev limiting would start at 7000 and apply a full cut at 7200.