CO2 calculation out of pH and KH levels are not very reliable. Most of the times it's less. Various things like nitric acids influence pH which make us think we have enough CO2. An example: The pH in my tank is 6.2 and the KH 5. According to the CO2 table all my fish should be dead, but even my discus don't have any problems. Discusfish don't tolerate more than around 45 ppm, so the difference between measured values and real values is around 50% in my case. 50% less!! So that could mean that you still have 20 mg/l, but also that it could be as low as 10 mg/l.... Dennerle sells a good drop checker which uses water form the tank to make the indicator fluid. That makes it more reliable. Or even better, use the fish as an indicator.
Dosing micro's wrong....well, if you dosed too much, that's no problem, if you had a deficiency, you should see discolorations etc. That's the easy part to correct.