Your water level should be controlled by your ATO float, so adding more water is temporary at best (eventually evaporation will level it all out).
First thing I would do is set your pump at the level you think it should be at. It's up to you, of course, but at 400 you're turning your water over at roughly 1x - which seems low to me. Fully-open, the pump you mentioned would have roughly a ~2x - which is still below Red Sea's recommended level (which is 790gph for the 350). I would start with your pump fully open unless there's a reason you want low flow in the tank.
Once you've got your pump set at the flow you want, close your primary drain 100%. It will be loud. There will be bubbles. It will make a mess. Now slowly open the valve - no more than 1/2 turn at a time. After you turn it, wait a few minutes. It will take time for everything to equalize. Things will get quieter as you get closer.As you get closer, turn the valve less and less for finer tuning. If it helps, the overflow chamber is made from mostly opaque acrylic - but not completely. Use a good flashlight and shine the beam through your tank onto the compartment and you should be able to see into it enough to gauge the water level.
Once the water level is within a few inches of the "emergency" drain, you should be making only the smallest of adjustments - always in the "close" direction (clockwise). If you go too far, re-open it a slight bit more than you last adjusted it, let it settle, then try again. The goal is to get the water level to be stable somewhere between 0.5-2" below the level of the emergency drain. It's not uncommon for me to have my water level settle right at the edge of the drain (until it appears that the water tension is the only thing keeping it from dropping in) and I've heard of others that run at this level intentionally. In general, I lean towards the water level being closer to 0-1" below the emergency drain; lower than that and I get an occasional gurgle.
Once you get your water stable, you might need to pull some water out (or add some in) in order to correct it. Your goal should be to add enough saltwater (of your desired salinity) so that the ATO float switch is just barely closed (look for water ripples from that direction while the system is running, or mark the water level in the ATO and watch for it to drop - though the latter is harder to notice, I found). Once you're there, you should be able to easily maintain your salinity. Otherwise, if you're over that volume, evaporation will eventually raise your salinity past where you want it.
Caveat: I've got a 525xL, so I apologize if there are differences in our systems I'm not aware of. The approach should be similar though, so I hope something above helps. Good luck!
