water height in overflow box

[sphelps]

usually the standpipe determines the overflow water level, and it should be just slightly lower than the tank water level as the closer they are the quieter it will be.

For the herbie style you don't use a standpipe and use a gate valve to adjust the water level in the overflow box. For this style you need a back up drain in case gate valve gets clogged or the flow rate changes. Usually the overflow box has two holes, one is left open from the bulkhead and the back-up usually just has a long pipe that extends near the top of the overflow. The gate valve is adjusted until the water level in the overflow is just below that pipe end.

I've always done the herbie style a little different and use a standpipe rather than the back up tube, it allows a little more freedom in adjustment as if water goes into the backup it won't be noisy. You can also get away with using a standard ball valve rather than a hard to find, expensive gate valve.

The water level in the tank will be about 1/4" -1/2" above your overflow box depending on it's size and flow rate.