I really would again advise against a valve in the drain pipe(blue)there is no reason to throttle this down unless you have a 'T' before the valve and then also routing to another section of the sump.
If you use this setup and throttle the drain(overflow box pipe) down,it can backup and flood,it only takes some crap from the tank or algae which was floating around to block it.
Is that overflow box made by you?
Old Chinese proverb:
Don't go to bed with ichy bum
wake up with smelly finger!!