Blue dream flowering question

So i am growing 6 blue dream under a 400watt mh/hps. They are about a foot tall now and due to limited space i can only grow them to 3 feet tall. I need some advice from fellow blue dream growers...at what height should i flower the plants in order to end around 3 feet? Any advice would be greatly appreciated.

Flower now!!! They will stretch 2 -3 times there size now so you might need to do some LST (Bending of the plants)

Thanks for the quick reply. I know blue dream is a very large plant. I was just wondering because i have always heard they will double to triple in size when flowered but my last and only grow did not even double in size when flowered. Thanks again tho.