Neil's relative approach is the wisest method using a pH controller, and still use it. You get the relative usage, the part that actually counts.

Like adjusting the bubble rate with a needle valve to hit that relative pH, you still use the same needle valve without the pH control to adjust CO2, you just use a bubble rate instead.

The bubble rate is relative a ball park amount as well.

The best methods we have for measuring CO2: the pH meter/KH reference and the pH KH ref solution drop checker.

Even those could be considered relative since we often add as much CO2 as we can without pestering the fish, that is different for many different species/tanks etc.

Regards,
Tom Barr