I'm assuming that you are referring to 20 clicks of rebound damping.
Before I ride a bike for the first time, I set the tyre pressures, levers, saddle height etc
Then...
I set the rebound to one third away from fast.
Compression damping to half way.
Then the sag to 30%.
Then ride and adjust to taste, one at a time!
Others may just ride it and only then adjust, but I prefer to shorten the set up process.
If you have not see any advice on how to set a good start point for rebound, the usual method is to keep it fully open ( anti clockwise) until you have set your sag, then using hand pressure quite violently push down on the bars and release quickly. If the wheel bounces with the tyre clear of the ground the rebound is too fast. Turn rebound clockwise a few clicks and repeat....keep going until you notice the wheel does not bounce off the ground. From there you can go a few clicks either way to speed up or slow down rebound to suit your riding style and preference. Personally I like my rear suspension fairly slow to stop it bucking on jumps etc and try to set my front to match or maybe just marginally quicker.