Why would it be hard to find the ideal co2 level if the light intensity were very low?

I'm assuming this question refers to the rate of photosynthesis.
Plants need both light and CO2 to photosynthesize (PHOTOsynthesis means using light to use CO2 to make O2 and other nutrients). If the light intensity were very low, the plant wouldn't have enough energy to photosynthesize much. 
You could have all the CO2 in the world, but if there's not enough light, the plant wouldn't be able to photosynthesize.
In short, it'd be difficult to find the ideal CO2 level if the light intensity were very low because the plant needs a certain light level to even photosynthesize. You could only find the ideal CO2 level if you had enough of all other nutrients, and CO2 level was the only variable you were changing.