How tall to veg?

Hello everyone. So I'm wondering how tall my plants should be before I switch to 12/12 light cycle. I have 4x2 space that is 7 feet tall. 400 w hps/mh with hood, 6" inline fan. I did a test with the light on to see what the temps were like. With the fan blowing, the air coming out felt about room temperature. It seems I can keep my growroom equal to outside temps, but with no ac it's not getting much cooler.

Since this is for personal use, I am only growing 2 plants (white rhino and nypd). I have them in 5 gallon buckets and will be feeding them foxfarms grow big and big bloom once they are ready.

I would like to know how tall should they be before I start flowering cycle. Also, I know it depends very much on a lot of variables, but does anyone have a rough estimate of how much bigger the plant will grow during the flowering stage? 1/3 the height at start of flowering perhaps (a wild guess on my part). My goal is to get 2 ozs per plant or more without sacrificing potency.

Thanks

keep a couple things in mind when deciding when to go to flower. height limitations, ability to control environment, wattage of light. of these, wattage is key to me. you don't want a giant plant with a low wattage light that won't be able penetrate far enough into the canopy. also, imo, you already have them in too big of containers. by limiting root-mass, you also limit height. with a 400 watt light, i wouldn't go bigger than 3 gallons. rule of thumb is one gallon of medium per foot of plant. i'd flip them at 6-8" if it were me.

Thanks for the advice. What do you think I'll pull off those plants in dry weight? 1-2 ozs?