That is a ball valve, which is much harder to set compared to the precision of a gate valve.
I have a feeling when you get those swapped out, you'll be able to dial this in correctly.
I seriously doubt you can set up a 'perfect siphon' that will always remain constant. Water changes over time, things clog up, plumbing slimes or even gets obstructions growing within the PVC, pumps slow down, chemicals as well as food are added to the tank, water temperature varies, salinity wanders, RO water is being added, and so on.
Better to set everything up to work within the standards of gravity and flow, and get your equipment dialed in to match those settings.
I have a feeling when you get those swapped out, you'll be able to dial this in correctly.
I seriously doubt you can set up a 'perfect siphon' that will always remain constant. Water changes over time, things clog up, plumbing slimes or even gets obstructions growing within the PVC, pumps slow down, chemicals as well as food are added to the tank, water temperature varies, salinity wanders, RO water is being added, and so on.
Better to set everything up to work within the standards of gravity and flow, and get your equipment dialed in to match those settings.