The depth for the air line would be near or just below the emergency drain, it is so if the water gets above it the first goes full siphon and “flushes” your overflow. The hole on top could serve the same purpose.Im designing my sump bean animal style for a 225 gallon and was considering a 2000 GPH pump. From my research the hole in the primary is not correct and your pump is way too big for your tank. Ill be curious to see what happens with your lower flow pump.
Ill add...I thought the small hole in the backup was supposed to have an air line connected to it and secured at a specific depth in the overflow (not sure where/what depth yet), no?