Thats pretty much impossible for me to do. I already have the largest bulkhead that I can get for the hole. And I can't drill it out anymore without affecting the structure of the tank.

I can maybe step up to a larger pipe ( 1 1/4" or 1 1/2"), but I don't know if it is a benifit considering the bulkhead restricts the flow.

I used the calculator that you showed, and it seemed to back up the size of the pipe and the flow rate. So So far it seems like a 1" pipe will work for me with a valve on the return.

You could also put a valve after the return pump to control flow rates if you don't want one on the drain.