Minimum Clearances when drilling an overflow

[lastlight]

When I said too close together I just meant vertical distance between drain intake and emergency intake. I'm not sure how some people tune their valves so exactly but I've always found the water level will creep slowly up or down no matter how careful I am. So I tune it so a tiny bit trickles into the emergency always. If the emergency was too close to the drain (water height above the drain was too low) it might suck air. All I meant but maybe I misunderstood your issue.

I'd also try and use the smaller hole as your drain before drilling though.