unless you want a lot of fish etc. to make a trip to the sump I would recommend an overflow box with "teeth" at the top to help keep smaller fish from getting pulled over the top with the flow. build your standpipe to be as tall as the overflow box( height of overflow box dictates the water level maintained in the tank) with your return elbow about 4-6 inches lower than the top of the standpipe. Also do not glue in the standpipe to the bulkhead, that way you can remove it to clean the overflow area from time to time.
Here is a great thread to explain the standpipe construction in detail.........
How do builld a standpipe for your Internal Overflow
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