As your using cfls you will be able to use more of the height you've got as the lights don't need to be as far away as metal halides or hps, which i think was what buster was getting at, correct me if i'm wrong buster. As a rough guide i'd say most sativas will treble in height from when you start to flower, so if you veg till your plants around 0.4m it should finish around 1.2m giving you 0.3m to allow for your lights and a little distance between your plants. You should try to keep your lights within 5cm of the top of your plant. For your first grow you might want to be a little more conservative and flower around 0.3m. I'm in the uk too, where did you get the 65watt cfls?