antenna cutting

I bought a Laird B132S mag base antenna for VHF communications on the 155mhz MURS bluedot and green dot freqs. My antennas overall height (the antenna to the part that unscrews from the mag base) is 23" inches. My antenna whip measured from the top of the spring to below the ball tip is 18 and...

I have a tram 5/8 wave NMO mag mount that came with a cutting chart. I am wondering where to measure from to cut the antenna. Top of Antenna to bottom of Antenna , or top of Antenna to top of base? The one I have goes into the base about an inch or so, thanks.