I think Ryan pretty much covered the basics you need to know in his reply to your other thread
As mentioned there, CO2 usage will be dictated by the amount of light you have over your tanks. There are many methods people use to determine CO2 levels with the drop checker being the most accurate at this time. Currently, the desired level for CO2 in a planted aquarium is 30ppm. Much higher than this and the fish begin to gasp at the surface, much lower and plant growth slows and algae growth begins. However, a few people out there do use lower CO2 levels successfully. It really can be a fine line trying to determining CO2 levels in our tanks given our current testing methods, especially if you want a specific number.
I'm not a big fan of not using a filter in my planted tanks but I have used power heads for water circulation. In a CO2 injected tank, you need some sort of method for circulating the CO2 rich water around the tank. This can be either a filter, power head, or pump of some sort but getting the CO2 enriched water even distributed around the tank is a necessity.