that may be the problem, not being able to adjust the overflow.
As far as I can think of the best way to adjust would be to turn the pump off, then set your tank and sump water at the level that you want, now take water from the tank and fill the sump to the top, wherever the water level is now inside the tank that is where you want the overflow set so that no more water is running through( just above the water line).
Set your pump to 150 and turn it on
theoretically that is how its supposed to work, when you turn the pump on, the tank will start to fill and start to run through the overflow right away and continue to circulate.
Let me know how it goes.
Kevin in Billings, MT
no tanks at the moment but going to have a massive build in the future