Light will determine how much Co2 will benefit your plant but if you are providing enough I have read as high as 1800ppm+ can be beneficial. Its generally accepted that 1200-1500ppm is ideal. If you don't not have the lighting to drive photosynthesis rates higher Co2 will do nothing to benefit you so there is more to that statement than just add Co2, like there is with most statements. Light is usually the limiting factor in photosynthesis when you can provide more light than can be used by the plant for photosynthesis then Co2 becomes the limiting factor so supplementing extra will increase photosynthesis and growth rates.If you don't have the lighting you will get no benefit from Co2 and that's an absolute fact.
You have to understand that once your supplying 1300 ppm that to make anything effective after that. Your going to be required to supply an amount of light energy that is now, getting hard to actually deliver.
Not only that but the curve in the relationship in the amount of light energy needed to that amount of gas over 1300 and above to actually be effective. Rises on a bell curve style of need to make that 1800 ppm you mentioned....Impossible to supply, and that's even for the professional commercial grower.... Those statements of 1500 ppm are out of the average realm of 98% of the growing community!
That 1800 ppm level was done in lab testing conditions only......Never to be used effectively at home or on the commercial market..It's simply the laws of science. At least at this time in lighting science....
What Co2 really does is replace the ability of the plant to effectively "grow" at normal growth rates that the light saturation point removes during the normal grow day. This light saturation point hits the plant earlier then many think. It's (the plant) cells in the leaves actually begin to change on the peptide level. This change is to actually
protect the plant from the intense light of day. This includes UVB protection.
This reduces active plant growth by around 30 - 33%+ per day.
At night, the plants reverses gears, and the cells change back to being able to use all the light energy they get.
Gassing will stimulate the plant to increase it's actions to negate the need to protect it's self. It then grows at normal rates, longer and you reclaim that 30% or so lost growth from light saturation points.
All C-3 plants have this "light saturation" point.. The droop you may see in your plants later in the day or right after lights out.. Is this "saturation point" showing you it's effect.