Since the main pond pretty much receives all of its water from the sediment pond drain I would think you would never exceed the main pond stand pipe level if you were just a few inches larger in diameter. The extra inches could account for straight rainfall onto the pond surface and some run off from the sides.
From an engineering standpoint, that is risky thinking. You don't design one failsafe assuming that a dependent system will never fail.
You hope to divert large influx events. If that system jams or fails to work how you hope and most of the influx passes to your main BOW, you must be prepared to deal with passing it across your main dam. If you overtop the settling pond, that's worse than a bad rainfall. A 20% capacity overflow would be overwhelmed quickly, then you're looking at emergency spillway to take it. There are guidelines out there to size systems based on maximum rainfall events. Spending money on a professional evaluation may be worth your while.
