I don't use CO2, but my understanding is this. The only time it is really going to be of grand use is when one has strictly scienced out and created a strain that you can calculate or know what it's best nute requirements are. I don't mean a good idea of what nutes are good, but hard science concerning your particular plants Nute uptake, howuch and so forth.
This is important and relates to your PAR results, which will vary from light to light and also height, which will affect both center zone par, and the lights distribution in the corners and PAR measured there.
Typically, around 900 -1000 is max a plant can use. Beyond this, more light is a detriment and the plant will suffer.
Now, if you have the previously mentioned info, concerning maximum Nute uptake and other Nute specific info on the strain, you may be able to adjust the nutes to the maximum useful level, which will be dependant on the maximum amount of light.
Once you go past this maximum amount of light, the plant will want nutes sufficient to use this extra light. However, at some point the plant can not intake any more nutes, and increasing light will increase this problem.
Now, assuming you are capable of the previously mentioned knowledge of your strains nute requirements ( I am not, and I would guess the number of home hobby growers that possess this level of skill are extremely few and far between) then my understanding is, that you can go somewhat beyond this 900-1000 PAR by adding CO2 to the mix.
I don't know of any hobby growers that have this level of ability and knowledge.
Not saying there aren't any, but precious few
Bubba
