Hi, The Mars Hydro got 132 True Watt, i would start with 40 cm and then look how the plant reacts, if it stretches to much its to far away, it plants are "dwarfing" ( building the nodes very close) its to near. Anywhere between 40-60 cm is the right range for that stage. There is no constant number because every LED is different and got different lumen. The Mars Hydro 300 is made for a growspace 60x60 cm / 24x24 inch in vegetation. For flowering is a smaller space recommended18x18 inch