I have 600w HPS and without light mover about 40cm is as close as I want it, with light mover you can go for about 25ccm, remember though that you have to find a balance as the close to the plant(s) then the more the heads will shade each other, I sometimes compromise and have the light about 45cm and then it catches all the girls> Put your hand under the light and move it down until it feels warm, but not uncomfortably so and thats about right. They are weeds at the end of the day and they will do what they do....dont stress or they will know something is wrong and grow up with behaviour problems and be excluded from canna school!!