More than anything I am curious about the ratio of natural sunlight vs artificial light - either by mercury vapor bulbs or UVB bulbs.
There is no real good comparison here. Like mentioned, you'd need to know the UVI level of your specific area, at the specific times of day that your dragon is outside. And for that you will need either the solarmeter 6.5 or the solarmeter 6.2.
Now personally, I don't like the 6.2 for indoor lamp use, but it is effective outdoors because we know the intensity of the sun, and we know how to calculate a UVI level from the mW uv cm2 that the 6.2 reads. But, for ease of use and simplicity, I'd go with the 6.5 as it directly measures UVI. Here is a link if you're curious on UVI vs. mW UV
Anyways, as you can see, you can't really compare. In the tanks if you follow the guidelines most agree on, they have the ability to expose themselves at their basking site to a UVI level of around 3-5~. Your area in the world, or your specific microclimate might never reach UVI levels that high. You might have days where the UVI level never goes over 2.
Simply put, we have a good understanding of the wavelengths of the UV spectrum that produce vit d3 in humans. And it's a safe (or at least the best we have at the time) assumption to think that these wavelengths are also the wavelengths that are needed in other creatures, dragons included.
What isn't known for sure is if there are other parts at play. Other things that the sun provides that increase this d3 synthesis.
I think most would agree that natural sunlight is best if possible, but also don't be fooled into thinking that your natural sunlight is enough, or good enough to fully mimic their natural environment or to meet their needs.
That being said, I think if weather permits, it's always a good idea to let them get some natural sunlight. Even if the UVI level isn't exactly where you want it. Sunlight provides much more benefit than just UVB exposure. But, making sure they also have what they need in their tank, gives extra peace of mind knowing that they won't be lacking anything.
So I'm sorry if this wasn't exactly the answer you were looking for. The reason you haven't' been able to find an answer is because one doesn't exist really.
Truth be told, we don't even know how much time wild dragons spend basking in their natural habitats. Not one study has been done that scientifically answered that question. Only anecdotal "watching" of dragons in nature has been done, as far as i'm aware. Of course it's because meters and trackers would have to be attached to wild dragons, that way we could monitor what level they were exposing themselves to, and for how long. That truly would be an eye opening experiment for everyone. And hopefully it's one that will get done soon.