The finished height really depends on strain, and their proximity to the lights. You should keep the lights as close as possable (without burning the plants) if you want them to be as short and dense as possable. However, with new clones using your 400w MH you should keep the light a good distance away (say 18 to 24 inches) so you don't burn the tender clones. Once they have developed roots, and started to grow a little, you can move the light closer. If your light is too far away it will cause the plants to stretch toward the light, and become tall and lanky. On average you can expect the plants to double or even tripple in size, from the point when you switch to 12/12 until harvest.