If you keep a piece of tap on the tank/sump it can be used to mark the water level so you can tell if it's changed much.
It can be possible to have to adjust the valves after a water change because you might change the balance of the head height. Say you remove 90 gallons from the tank and then you put 100 gallons back in. You now have ten extra gallons in the tank and sump that will change the water height in either, or both places. This will change the head height that the pump has to pump the water up, only slightly, but when you're talking about such a fine tuned setup, it can be enough to cause the pump to push too much water and increase the flow in the emergency overflow, or push too little and cause the siphon to be lost on the primary overflows.
It can be possible to have to adjust the valves after a water change because you might change the balance of the head height. Say you remove 90 gallons from the tank and then you put 100 gallons back in. You now have ten extra gallons in the tank and sump that will change the water height in either, or both places. This will change the head height that the pump has to pump the water up, only slightly, but when you're talking about such a fine tuned setup, it can be enough to cause the pump to push too much water and increase the flow in the emergency overflow, or push too little and cause the siphon to be lost on the primary overflows.