I don't know how accurate it was and the guy sells avian lights himself but there is a youtube video of a guy showing the output of bulbs. In the video he shows that there is very little to no useful uv light that actually makes it down to the bird. I know this has very little meaning here but I would not worry so much about it. Just follow what the manufacturer suggests and your birds reaction.
I keep a lot of toys on top of Pip's cage so that gives him a way to get out of the light but he doesn't. He spends most of his resting time on the high perch. Well he spends a lot of time on the top of his swing now he even sleeps up there. I do not know how hanging on the side of a swing can be a nice place to sleep... haha
Now the reason I went with 10 hours is because I had read it someplace in my research. I am not sure where it was at now but it was likely from an avian vet article of some kind and they recommended 10 hours. This and Pip does not get any natural light because my windows are older and a little drafty so his cage is far away from them.
Here is the link to that youtube video I talked about if anyone is interested..
I found that article of the doctor talking about light if you want to have a look..