While I was looking at the usual chart relating KH - pH - CO2
I found that if I wanted a 'target' CO2 level of 15mg/l this always
resulted in a drop of pH of about 0.7 from the no-CO2 equilibrium
(atmospheric) level of 3mg (is that right ?) to the pH at the 15mg injected
level, for all KH values between 1 and 8
sorry for poor english, did that make sense ?

Doing the same for a target of 25mg the ph drop needed was 0.9 for all/any
KH water.

Similar, for a target level of 30mg/l required(resulted in) a drop of about
1.0 in the pH for all KH waters from KH=1 to KH=8 (ie. all the KH's that I
examined )

In other words, I dont need a KH test kit, all I need is a pH test
measuring the non-carbonated water and the carbonated(injected) water.

Am I losing my marbles, I've not seen any refs. on the internet to this,
they all require me to test the KH to know the mg CO2
What am I missing ?