Right guys , just about to start my next batch of ...

Hello @Grass_slinger ! I don't want to disappoint you but co2 bags will never make any significant difference at co2 ppm levels especially if you are exhausting the room and its not sealed. In general you must have in mind that when you raise the co2 ppm you also have to raise the ppfd of the grow room. For example at 1200-1500 ppm of c02 you need around 1000-1300 μmol/m2/s. For most of the HPS lamps that's about 800 w/m2. Also above 450-500 ppm of co2 during night can become toxic. Usually at large operations they bump up the co2 only at third week of flowering until the end of the sixth week. Happy growing ! 👊

What I do is have the grow box inside my room, so to take advantage of the high CO2 level present in a bedroom during the night, while having the 12 hours of light during flower happen at night time. You have to be sure that the light at canopy level is above 85.000 Lux so the plant can use the high CO2 levels, else you will just waste money.