How to use bubble rates
I suggest that you make a chart of the bubble rate and the ph at that time. Convert that to CO2 with your normal KH (assuming it does not vary much).
Now, you can *roughly* tell the CO2 level from the bubble rate -- for your tank, assuming everything else is pretty much the same; KH, fish load, waste load, gassing off losses, reactor efficency, tank temp, and so on.
What you want to know is what is the lowest bubble rate that is still giving you a good CO2 level. So, when you see "X" bpm, you know it is time to change the mix in the DIY. That number might be 15 bpm, or it might be 5 bpm, it all depends on what happens in your tank and how much a swing you want to tolerate in the CO2 levels.
With the ladder, you can go one step further and count the bubbles visible at one moment. I found that number of visible bubbles was very close the the bpm rate and so I knew if there were "Y" number of bubbles or less visible at one moment then it was time to refresh the mix.