You need to observe the fish with the current level. They should show no sign of stress, gasping, lack of appetite, etc...That's why I mentioned shrimp. A couple of mine were trying to escape on just a double dose of easy carbo(because they weren't used to it too though, so hence gradually increase the level rather than all of a sudden) Shrimp are faster to react, tolerate less CO2, so they are a good guide. While adjusting the CO2, spend more time around the tank and outgass it if needed, roll back to the previous value.
The drop checker is a helpful indication of approximate level, but it isn't error free. If plants are showing you they don't' get enough CO2, then they don't. They'll basically consume the CO2 before it becomes dangerous to your fish. If the drop checker is yellow and your fish aren't bothered by the CO2 injected, then what is the conclusion?
It's a good idea keeping an eye on everyone with the increased level to see if it's ok. At night time it's a separate story. See if the fish go at the surface at some stage, etc...and use an air pump on a timer for some time if you have to. Or have the CO2 on a timer instead. That's just my logic. There maybe a better way.
You can still use some surface aggitation for O2 to keep the fish safe day or night. That just means that you may need to inject more CO2 but on another hand it will keep the O2 levels higher too.