the equation is basically this... how close can you get your plants to the lights without negatively effecting bud quality.. on a 600 its around 14" with an AC hood. So that is the end all be all height. I said 16" to be on the safe side. Then you have to factor in how much they will stretch and this is the tough part and depends on part and really depends on strain, how vigorus they are and how many branches you have..more branches= less stretch. I found that having the buds about 12' above he screen works best. So I said 12-16"
the trick is to stop training them at the point in the stretch that they still have some stretch left in them so they get out and above the screen. I can't stress enough about removing the lower shit. I would remove it now to avoid stress before you flip. All energy put into producing the lower buds is wasted energy. the scrog will be very thick and you will get next to no light down there. the buds will be fluffy, yellow and worthless. Most of the leaves will yellow die and make a big fucking mess anyways. in your size grow not a problem. But when you have a 7' x10'scrog and you have to scoot on your butt with your head held down and brushing against scratchy leaves it becomes a whole new PIA.
Post some pics!