Start really, really slow - it does not take much to kill everything in a 10g. If you bump the rate up, plan on staying home for 2 - 4 hours watching your critters. Don't trust drop checker - your fish and shrimp are the only real indicator if you went too far. Keep an air pump handy in case of emergencies.
The co2 saturation is dependent on the gas exchange. In other words, if your surface agitation slows down due to filter getting clogged over time, the co2 concentration goes up. If your co2 is at the limit, that can be enough to kill the tank. I did that - a lesson painfully hard learnt. Same goes for a sudden drop in temperature.
Some fish addapts to higher co2 levels over time. Tollerance levels vary between species and individual fish. If you adding new fish or shrimp to a long running tank then turn co2 off, do a large water change and start over.