The adjuster has over 40 clicks but only 21 are used in setting the rebound with the forks assembled.The important point is that all 21 are available when the forks are reassembled.If you use the 12-18mm measurement the damper rod holder nut may not be tight against the spacer. Do not over tighten the nut as it can warp the spacer tube, it just needs to over finger tight.
From my experience of doing a number of forks, the option I found best was to wind the adjuster in full, now wind it out full counting the clicks.Then wind the adjuster back in exactly half way and reassemble the forks. The last set of forks I did had 42 clicks total from full in to full out on both sides.This method ensures you have sufficient clicks to se the forks up properly.
Treat both adjusters as individuals, I have come across some with 2 clicks more on less on one side.When the forks are assembled wind the adjuster in full and work back from their to get your required setting.