what ever space you can get away with, remember your light when at highest and can no longer raise it, and the plant height. As for scrog, do you plan on scrogging all the way around your grid or just a few colas being held up? If you plan on scrog all the way around you can minimize your tops of your plant but will take a lot longer. I would scrog all grids then switch. By the time you switch you can probably keep your tops only a few inches above the grid. there fore you must just kinda calculate your distances. The light should be 4 inches above the tops, so when in full bloom when the plant has stopped its growth spurts and is focusing on the buds, you can predict just several more inches of growth. depends on the plant. Im doing a cfl grow and in about a month into flower i still need to raise my light maybe an inch every week. so say 5 inches to be safe of moving the light up plus the 4 inches between the light and plant.
The grid i think on your plant can start maybe a few inches above her. So when she pokes her leaves into the grid you can trim all the bottom leaves off and let her focus on new growth inside the grid. You have to make your own measurements because you will see the growth of the plant plus every strain is different. but keep in mind a distance of maybe 10 inches you need when flowering starts. if you see too much growth you can supercrop the high points by pinching the stems side ways with out breaking them in half.