Drilling a overflow

[Masoniac]

I just bought a 75 gallon tank today of course it's not reef ready but it was a deal I couldn't pass up. I was wondering what size the hole should be for the overflow?

 

[scuba dude]

I am working on a 55 gallon and installed two 1 inch bulkheads and that uses 1 3/4 hole. Would depend on the size bulkhead you would decide to use.