Buy yourself a nice cheap handheld temp gun....tell you exactly how close you can put your light to keep leaf surfaces under 80 degress....which I am told is optimum.

PS...I'm told by the same source....(who knows his shit btw) that that is how the "pros" do it......with a light meter and temp gun which will tell them max watts per sq foot so I'm not sure there is "one" right formula for plants.