3 weeks veg is very short.
You don't need to monitor ppms with dirt.
CO2 is something only a really experienced grower can see a difference in yield using.

Under perfect conditions you can get 0.5g/flowering room watt/month
So shoot for 600g/crop. That's over a pound.

BUT. and it's a big but.... That's if you have everything tweaked in perfectly, including vegging for a longer time.