The answers here are lacking that luster. I wish there was some standard systematic way of determining distance by first setting aside all variables like temperature, enough water, and nutrients etc. Let's just say the wavelength of light being absorbed is enough energy to excite the P680 and P700 Photo sensors to activate the electron transport chain, whereyou need iron calcium and magnesium, where the incoming energy is the right energy where E = hc/Lamda I still would need to add the all the sensors the plant had on average with some score factors to account for things like nodal spacing and leave width to relate the exact distance of an LED for a specific plant. This could take years of research. just like 12" - 36" its in there somewhere.
