I think the best is to determine the ht. of the light. You'll want about 2' of clearance for the light from the screen. (250W) more if higher Watt HID. So you can move it up as vertical ht. during stretching. It would depend on the type of HID your using. I attached a chart for HPS light distances to help you figure it out.
Also take the ht. of the pot. and add at least 6" to that for plant growth to the screen.
This is for an NL grow tho. Prolly need more ht. for other strains as NL is really short.
thanks im using a 400w HPS. To put the question another way. Whats the shortest lengh the plant can be before i can start training? i can adjust the light to any height, so thats not a problem. I am also growing NL