When you adjust the valves, start with them wide open. If the overflow won't handle the full volume of the pump, throttle it back until it will. Then give it a minute or two to equalize. If the flow rate looks good then move on to silencing the overflow.
For the overflow, you need to start from the wide open position and make very small adjustments. You will hear the pitch of the gurggling go up with each adjustment. When the pitch changes in larger degrees, then you are getting close. At this point, you will need to give the system more time to settle back into equilibrium before making another adjustment. (You can also find the approximate valve position by closing it until it is silent and then backing up a little.) When the overflow is completely silenced, you will need to watch the tank for a while to make sure you adjusted it correctly. Also it is a good practice to open the valve slightly to make sure that it can operate silently at a more efficient position.
Most importantly, you need a second overflow even if this one can handle all of the flow. The second is your back up in case something obstructs the first one. Its intake needs to be slightly taller than the primary overflow. When you get the primary adjusted, measure the water level above its intake and then shorten it by that amount. Use the current measurement as the backup's height (that will put your water level where the top of your overflow currently is)(you may want to tweek the surface's position).
