"...Once you know your KH..."
And therein lies the problem as this is easier said than done.
The BIG problem wth measuring CO2 is that of accurately measuring pH and to a much greater degree KH. This is where in-tank permenant CO2 testers come in because the reference liquid used uses a VERY precisely known KH solutution (which if prepared by yourself is NOT dependant on measuring KH when preparing), a pH indicator liquid is then added such that it turns a certain colour at a given level of CO2.
If you have 'upped your CO2' then you should certainly expect to see either pH lower or KH raise:
CO2 (ppm) = 3 x dKH x 10^(7.0- pH)
This is not a reversible formula: Don't think that by fiddling with KH or pH will therefore 'make' CO2 go up - the only place that CO2 will come from is .... CO2.
I'll pitch this one in for completeness: To quote VaughnH of the BarrReport....
The pH of 1.0 reduction method over 24 hrs, using the pH of "degassed" water vs. the water in the tank, only works if you know the resulting ppm of CO2 in the "degassed" water. A 1.0 reduction of pH means there is 10 times as much CO2 in the tank water as in the degassed water. But, a 1.3 reduction means 20 times as much CO2. A BIG difference! And, the ppm of CO2 in degassed water can be between 0.5 and 3.0, depending on how long you wait and other factors unknown to me. So this method may be easy to use, but is far from an accurate one.
Andy