Bubbles per second are really only useful for knowing when there has been a change in CO2 output or to set the bubble rate to where it used to be after having played around with it. It says nothing about your CO2 levels in the tank.
There are too many variables to use bubbles per minute effectively, it's like saying that 10 gallons of gas will get you 100 miles of travel without considering the driving conditions, engine size and weight carried.....
Here are some variables you need to consider:
- If you use a less than perfect method for diffusing CO2, you will need more bubble per minute to maintain the same level of CO2
- CO2 is easily lost due to gas exchange. The amount of gas exchange occuring in your tank will depend mostly on the amount of surface agitation created by your filter's return.
- Temperature of your water will dictate how much dissolved gases it can hold and the ease at which CO2 is dissolved into the water.
- Plant uptake will also play a minimal role in determining how much CO2 il left in the water.
So as you can see, running 10 bubbles per second through an air stone is probably going to give you less CO2 concentration than 1 bubble per second through a powered reactor. Infact in my opinion, controlling surface agitation and diffusion efficiency is the best way to control CO2 levels for DIY setups where CO2 output can not be adjusted.
In conclusion, measure your KH and PH, then use the table to know how much CO2 you have. Once you reach the desired amount, you can note how many bubbles per minute are required for YOUR tank to maintain such level. Should you change the amount of surface agitation or method of diffusing CO2, you will need to re-calculate your required bubbles per minute.
Here's a short article I wrote on DIY CO2, hope it can help http://www.gpodio.com/diy_co2.asp
This is one of those cases where you "bubble milage" will certainly vary from others...
Hope that helps