Yep, that's it.
Easy to do yourself - just a couple of 22 1/2 degree 8 inch joints and 8 inch tubing - cut a 3 inch hole in the joint at water level and that's about it. When the water level reaches the hole it will syphon the water from the deep area at the dam.
Guess your 12 inch tubing might work - we used 8 inch and it handled a 9 inch rain with no overflow at emergency spillway.
