a scrogging question

ok im in the middle of setting up my cab i have a 3x3 scrog that i will be doing ....i will be growing good old norther lights and was kinda curious as to what is the average length the the colas will stretch from the screen.
i will be using a 400 watt agromax hps air cooled.

so what i really wanna know is how much height above the screen will the buds normally grow.so i can see if i have enough height were my light is placed.

thanks everyone