Roof valley clearance

Roof has 9/12 gable with 5/12 cross gable. Using 24" metal valley over 36" underlayment. Is there a standard clearance from the center to the comp edge?
Drain area from 9/12 to 5/12 is about 2.5 SQ. I live in central valley, Ca. Annual rain is around 10", sometimes short period of heavy rain fall.
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