What determines water level?

[VSingapore]

Got a second hand set of Elos midi and once I got it all up and running I found out it’s soooo noicy!! Overflow that is. Driving me mad!!! Decided to modify to a herbi overflow, so in the process of slaughtering the set Question, what determines the water level inside the tank?? I need to see if I need to raise my overflow inlet…

 

[lapin]

It usually set by how deep the overflow box teeth grooves are