This chart is a really good way of getting an estimated reading of your co2 levels, it compares your ph and kh and gives you an estimate of what your co2 level is. Another way is if you know your constant ph level, when you add co2 you want it to drop a full point, example - before co2 comes on my main tank sits at around 7.2, once co2 has reached 30ppm (optimum level for plant growth vs health of fauna) it drops to around 6.2. Then over night it degasses and bounces back to 7.2.