Coralline will probably be your first big consumer.
When you cannot count the drops anymore, you measure the effluent in ML. Right around 90, or so, they get hard to count... so you count out 60 or 90 drips (just pick one) and figure out the ML... and you do this 4 or 5 times and figure out the average. Then, you use the MLs in your ratio.
If you ever get to the point where you cannot count CO2 bubbles, then you probably have too much coral on your hands. You can still count 120 a minute pretty easily. I have never really gotten above 30 or 40 and I have had SPS colonies that would need fragged every month to keep from growing together.
You can also test the PH of the effluent. It should stay the same as the ratio goes up.
In reality, you get the hang of it and can just look at it and see. It is kinda like setting the timing or adjusting valves on an old car - once you do it enough, you just know.
Once you CaRX, you never want to go back.