I suggest getting one of these, or something better if you afford it.
I have one similar and it's amazing to see where the light is actually brightest; I have found with HPS bulbs it usually shines brighter on one side....?
See link for LUX light meter
http://www.ebay.com/itm/Display1-20...cd-/171187049100?_trksid=p2141725.m3641.l6368
Peace, beware if you do buy one as you will be constantly tweaking light position/height...
Yeah me too....I have to thank you again. I love my LUX meter. To me it is a PRIMARY tool for a project.