I set mine by starting with the valve open, then close it slowly until the level rises just enough to just trickle a little down the secondary drain. Anything more than a trickle, then open it just a little. It might take a few adjustments, and every time you do, let it sit for a few minutes. Eventually, with just a tiny bit of water down the secondary drain it should be silent, and self regulate to a constant level, keeping your salinity stable (if you're using an ATO).
A screen over the primary drain should keep out any intruders (snails, other inverts), but don't screen either of the other two drains.
If the level starts to drop, it could mean your return pump needs cleaning.
Periodically, like during a sump cleaning, fully open and shut the valve a few times to knock off any accumulated gunk. The valve can freeze up if it's not worked open/closed once in a while. While you're at it, use a pipe brush to help keep things free flowing.