Example:
#1
Your skimmer box is 12x4 inches.
You want to flow 1800 gph.
The water level would probably be above the teeth in order to keep that much flow going through the overflow box.
#2
Your skimmer box is 24x4 inches.
You want to flow 1800 gph.
The water level may just be above the bottom of the teeth because it doesn't take near the
depth of water to get that much flow because there is more
surface area for the water to flow from.
As far as the 1 1/4" bulkheads, here is just one place that I found that has them. There are lots of places you could order them from if you want to use this size bulkhead.
Inexpensive Bulkhead Fittings, 1/2"