Not everyone has a Dremel and scoring the circumference of the whip with the edge of a metal file or just filing a notch in it and snapping with pliers works fine. The stainless steel used in most whips is very hard and it will snap like glass when scored.
prcguy
prcguy
I would suspect the spring has some additional series inductance and stray capacitance which affect the overall rod length somewhat. If this were less an art, and more a science, Larsen would simply ship a "CB" rod cut to channel 19 and there would be no market for VSWR meters.
Cutting tip:
1) Use a Dremel tool with cutoff wheel to scribe around the circumference of the whip where you want to cut.
2) Use a pair of vice grips or electricians pliers to "snap" the rod on the scribe line.
Make cutting easy so you can make many small iterations without frustration.