I've seen anywhere from 4-12" suggested. In reality it depends on your own grow, such as growing medium, light, space available and such. Gotta kinda test it out yourself to find the right one, you can also do a two stage, one net at X" height and another one higher at X" of height. I personally prefer to start low, 4", that way you can begin training asap in my setup and speed up total time spent. At 4" i need to then either raise my net or build a second one to optimize because I am limited on space when I train, so it varies by the space you have and the time you plan to spend training.