Robertsanswered grow question 2 years ago You would really need to aquire light intensity meter. Photone app let's your phone work as one on the beta version anyway. Then you will just have to test at different intensity amounts. Heat is also to be considered in extended hours of operation.
I always keep my light at the top and just use the intensity dial to change. Because you do want a good low 80's F in the grow space while operating. If plants are smaller I prop them up on crates or whatever to move them closer to light if needed.