A couple things. If the light unit doesn't have an integrator built in, you could use a Metrolux timer which is an external integrator which can be attached to any light source. I use Metrolux timers on my UV unit and my cold light heads for perfect repeatability. I don't know if they are still made, but if not you could probably find them for sale used. The Metrolux II is the better unit.
Also, if you do the test holding an integrator light-reading cell above and below the glass for comparison, make sure you know what UV bandwith the sensor is reading. If it reads a braod range of light, you're not getting a perfectly accurate reading of UV light loss since the UV light loss is proportional to the total light loss. As others have said, the difference is pretty small anyway, but the best bet comes in printing.