I think coverage is better, catching as much of what ever light u have is certainly beneficial. I have a 3 way multimeter that test light, soil and ph. I turn it on the light selection and although I dont know how to read it accurately, I just wave it all over my grow and am satisfied when it is 3/4ths to all the way pegged everywere I put it, seems to work pretty goodI mean it'll do until I get a par meter.