1. Full siphon - you don't even need a stand pipe if you don't want it. You can put a inverted U (2x90) if you want and have space for it and/or add a screen but I feel there is no need.
2. Open channel - should be about the height of the weir or at least as high as the bulkheads between internal and external boxes. This will minimize splashing (and salt creep) and related noise as the water flows into the external box. You should add a "T" with a 90 degree elbow on top of the standpipe. The T gets capped on top and a small hole is drilled into the cap. This small hole allows air to escape and if the water goes above it, hole gets sealed and that secondary drain will go full siphon.
3. Emergency - Can be 1 inch shy of top of the external box.
The whole point is to use the valve at the end run of the full siphon (right by the sump) to dial-in the flow so that the water in the external box little more than trickles into open channel drain. This will provide for absolutely silent setup.
If you have a return pump that allows you to dial it by tiny increment (Abyzz/Royal Exclusive, etc) you can also do minute adjustment on the pump's controller.
The setup I created on my new build that everyone said would be hard to dial in (2 external boxes) is absolutely silent! If you follow what I stated above yours will be too.
