Need advice for Sativa Scrog grow box

Im about to do my first scrog grow (3.5' x 2.5') with 400 w mh/hps lamps, with cool tube. The screen will be about 12 inches above the medium.

It will be hydro, and I want 5 clones of a sativa strain, most likely purple haze (70-90 cm tall according to http://www.marijuanastrains.com/purplemarijuana.html)

Im in the process of designing my grow box, and i want to keep it as short as possible.



Can anyone with experience of scrog growing sativa strains give me some answers/advice on the following?:

If I have the 400w lamp (w/ cool tube) about 18 inches above the screen, will the light stay at this height for the entire grow (up to harvest)?

Or would I need to move the lamp upwards by how many inches the colas are growing above the screen? (example: 18 inches + 4 inch colas above screen = 22 inches from lamp to screen)

How tall would colas similar to this strain get, and is 18 inches a good distance for my light system?



I need this info so i can build my grow box to a reasonable height. I want to have as short of a box as possible without decreasing my yield. The final heighth of my grow box will be: 20 inches (heighth of hydro tank) + 12 inches (medium to screen) + max distance lamp will be from screen

you need to maintain about 12 inches without a cooltube, less with. Ultimately it will depend on how much your plant grows above the screen.Some people say that with a cool tube the plants can touch the glass, but that wont give you the coverage you need. Best to play it safe and allow an area of margin and be able to move your lights