I've never heard the co2/nutrient ppm correlation theory before, and don't quite see how it makes sense, beyond the general sense that every aspect of the environment needs to be optimal to receive maximum benefit from other aspects. That doesn't explain the exact correlation though, which just doesn't sound like it is true.

The best thing to do is put your extractor fan and your CO₂ regulator on timers, so that they alternate, since there is no benefit in releasing CO₂ when the air is being sucked out of the room. Turning the fan off for fifteen minutes every hour is a good formula, I think, and that should help alleviate concerns with the smell leaking out, since the fan would still be running most of the time, at a regular interval. While you don't want to be extracting the air from the room while releasing CO₂, you will want an oscillating fan blowing air around the room, so that the CO₂ is circulating around the plants (it is heavier than air, and it isn't the roots that need it ).

I think the standard level people aim for when working with CO₂ enrichment is 1500ppm. Anything over 2000ppm could inhibit growth. CO₂ enrichment will only happen during the day, because its needed for photosynthesis to take place, and photosynthesis doesn't take place at night (even though the plants still grow at night, due to light-independent reactions). As long as you are providing enough frequent air exchange, it isn't necessary, but I'm sure there's a difference 300ppm and 1500ppm.  

The thing you need to keep in mind, though, is that it isn't just about CO₂, but really the relationship between your CO₂ levels, the temperature, relative humidity, and light intensity. For example, if the temperature and light intensity isn't high enough for photosynthesis to be happening quick enough for it to actually need the extra amounts of CO₂, then there's no reason to supplement. To quote from this, which seems to be a great resource on CO₂ enrichment:

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Standard growing conditions typically include concentrations of CO2 at 300-500 ppm, temperatures between 65-80°F, and relatively low humidity (20-40% rH). Studies have shown optimal growth and yields at 90-95°F, 1,500 ppm CO2, 45-50% relative humidity, 7,500-10,000 lumens/square foot of light, and vigorous air movement both above and below the canopy. CO2 enrichment under 80°F, under 7500 lumens/sf, or above 50% humidity is not recommended because plants will not be conducting photosynthesis quickly enough to benefit from the enrichment.

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I've never heard the co2/nutrient ppm correlation theory before, and don't quite see how it makes sense, beyond the general sense that every aspect of the environment needs to be optimal to receive maximum benefit from other aspects. That doesn't explain the exact correlation though, which just doesn't sound like it is true. 

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