You don't necessarily need 1500ppm at all times.. That would be ideal, but costly.. Most ppl turn the fans off, and dose to 1500ppm for a few minutes every couple hours, and cooltubes with isolated intakes and exhausts make things alot more efficient since you can tone down the grow area exhaust rate and still expell heat.. I know you're in a dungeon though, and can't dedicate extra ducting in/out..
CO2 meters are EXPENSIVE compared to any other meter in this game.. They do sell little disposable test strips or whatever, but they aren't really cheap either.. You just need to figure out how long it takes to raise the desired amount given your space (at the plant height), then use timers from there..
Some rough math:
1lb CO2 = ~10 moles of CO2..
10 moles of any gas will occupy ~8cuft at STP
So diluted to 1500ppm, 1lb of CO2 will create ~5200cuft of ideal plant air.. Actually about 6500cuft since only about 1200ppm needs to be added to reach that, and a 20lb tank should yield about 130000cuft of ideal air over its life..
IIRC, your area is roughly 75sqft (600cuft??), so that would be ~220 total air exchanges if CO2 weighed the same as ground level air..
But this is where need for a meter comes in since CO2 is heavier than air (its specific gravity is ~1.52), and you exhaust from the top.. Its tough to say how much gets exhausted.. Its safe to say that it would last quite a bit longer since exhaust is at the cieling, but there could be a long term buildup differential each day.. The more interior air movement you have, the less buildup will occur obviously.. You wouldn't want that circulation fan blowing it up at your exhaust, and you wouldn't want each cycle raising the level by x ppm unless you had planned on that, and know you'll be staying under around 2000ppm .. This is really kind of analogous to that hvac chat we had in your journal..