You'll need to unbolt the master from the booster, leave the cap and lines alone, pull it forward enough so that you can measure the tip of the pushrod to the mounting location of the master on the booster's surface/face, I'd recommend a set of Verniers/dial Calipers...then measure from the bottom of the inside piston bore of the master to it's mounting base/face/flange that would bolt on to the booster's surface, the do the math based on the values you have, either lengthen/shorten as needed, .030 minimum to .060 max for Mopar applications

be sure to grip the knurled/splined portion of the pushrod with pliers/vise grips, and turn the hex headed pushrod tip in/out for the desired length

Mike


FYI if your not familiar with using Calipers to measure the bore of the master, you use the section/bar that slides out of the "rear" of the Calipers and the "rear" portion of the main caliper frame to establish your measurement ( the DEPTH ROD as described in the pic below