This is going to help you tremendously. looking for a 3-4 UVI range. His back should be hitting roughly a 4 on a brand new bulb. The bulbs have a burn in period of a few weeks so that UVI at the back will reduce a bit after burn in. Let me know if you have questions when you get your solarmeter.I also finally bite the bullet and got a solarmeter! So I can know for sure.