Starting from the beginning: You use dip sticks to measure both pH and KH. Those are not very accurate, at best. Then you are just estimating that your KH is between 180 and 240, so you picked 210 as the number, but in reality it could be 170 or 250 or anything in between. So, forget the 17.9 conversion, and just use 20, which is easy to do in your head. Your KH, in degrees, is then likely to be between 9 and 12. Now, you are again estimating that your pH is 7, but it could easily be 6.8 to 7.2. So, if you use the table, determine what those extremes give you: pH 6.8, KH 12 or 57 ppm, and pH 7.2, KH 9 or 17 ppm. So, if your tank water contains nothing other than carbonates and CO2 that affect pH and KH, then you probably have from 17 to 57 ppm of CO2.
That's one of the reasons why it is impossible for us hobbyists, with only test kits for KH and pH, to measure how much CO2 we have. You can be sure you have some CO2 dissolved, and, since the fish aren't floating belly up, you probably have less than 57 ppm. To go any further with CO2 you would need a way to slowly adjust the CO2 bubble rate, so you could observe the plants and fish for several days for each adjustment, to see what effect it has.