whe do u need to start co2???????

hi, I was wondering when to start C02? The trees are in the 3rd day of flower and I didnt use co2 for vegative. I have a tank with fuzzy logic controller and the ppm in the room now is abt 350ppm and i have a temp of 84-87 degrees under 4 1000w hps in ebb n gro. My humidity stay around 30%. Should I start the co2 today? thanx

I usually start it around half way (3-4 weeks) through flower.....