The drop checker is only a guide just like your bubble counter. Your fish will tell you when you really need to turn down your co2.
Give it a slight turn up periodically though out the day until you see your fish surface. They are looking for air because of all the co2 in the water. Then just give it a slight turn backwards.
By doing this you will have as much co2 in the water as possible without harming your fish. This is important because your plants will not be competing over co2.