This is a really difficult question to answer, even with some understanding in plant physiology.
Co2 is only a limiting factor when all other variables are sufficient. With indoor enviroments, when a grow is well managed and healthy, light is usually the limiting factor before Co2 concentration. Studys show with current ambient Co2 concentration that outdoor plants often reach the compensation point (the rate of photosynthesis exactly matches the rate of respiration) daily when light intensity climbs to high levels. Greenhouse growers with feedback systems fluctuate co2 concentration with respect to radiant intensity. When the compensation point is reached and quantum yeild goes down (photosynthetic efficiency), co2 concentration is increased to maintain maximum quantum yeild.
So when you would want to supplement with Co2 would be at the compensation point, so that the photosynthesis rate and efficiency increases along a linear scale with respect to light intensity. Outdoors this is very simple to work out, as light intensity is linear in all directions. So the only thing you have to monitor is radiant intensity. Indoors however, not so easy.
Because artificial light is not uniform and we are able to manipulate it, it becomes more difficult to judge where one should add co2. As previously said, the compensation point dictates when co2 supplement will become beneficial. But because we have control of the light source, this could depend on what height a light source is. If a light source is close and so radiant intensity high, a plant could be forced to reach its own compensation point and photosynthetic efficiency would go down. Adding co2 at this point would be beneficial but unpractical. This is because getting light as close as possible does not increase "total" photosythesis. A light source has a specific radiant flux, which is the total energy a light source radiates. This radiated energy does not degrade (excluding specific variables) over distance, it only decreases in density because of more available space. What this means is that, as long as all light is reaching the grow, the amount of "total" photosynthesis is unchanged. This is where the judgement of when Co2 should be added is difficult.
At ambeint co2 concentrations, cannabis has been shown (chandra 2008 study) to reach its compensation point at around 500umls. This is actually similar to other high terrestrial species of plants. Beyond this point quantum yeild decreases, until it reaches a saturation level of around 1500-2000umls and photosynthesis rate actually decreases. At 1000umls quantum yeild levels off, this is about where any increase in light returns very little in growth. With indoor lights and ambient co2 concentrations, you would typically want to have light intensity at the canopy between 500-1000umls. Where exactly will come down to the type of light you have and the type of coverage from your refector or optical lenses (LED).
Where deciding when to add co2 suppliments indoors, will come down to two things. The radiant flux of the light source (total emmited power) and the footprint control. When more powerful lights are used, it gets harder to control the intensity at canopy. The further you raise your lights, the wider the distribution of light will be, this will cause reflection losses or non conctact (completely misses the plants). So the point of where co2 should be added, is where the power of the light exceeds the capability to maintain ideal intensity and light distribution. Now i understand that is quite unclear, unfortunatley its the best i can explain it. As i said, its really difficult to answer.
iv read her articles she's very smart and knowledable but I can't find a. Saturation level lol hope the weather gets better for ya 
