SCROG screen height?? 18litre pots

I have a 9 x 18litre wilma pot system under a 600 watt, will be only using 5 of these pots. Setting up a scrog but cant find out the distance from the pot to the screen. Anyone know?
I have 57 inches from light to top of pots. Read I need 20+ from light to screen. Is 37 inches to much from pot to screen for 18 litre pots?
 
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