A 400 might be fine. Get a dimmable ballast so it can be dialed back if needed. I use 2 400 watt hps that have a built in dimmer. Mine are at 100% because I didnt see any signs of light stress. You plants will tell you what they need. You have to learn how to read them. I originally started from seed using 23 watt CFLs and I had lanky spindly plants. I upgraded to 54w HO fluorescent tubes and the plants loved it. Proper height, beefy stalks and larger leaves. I tried to flower with the same light and had great quality smoke, however the buds were light and airy, not dense like they should have been. I upgraded to 2x 400 watt hps and have had great success.