Well, Proviron isn’t DHT, and so any studies regarding DHT aren’t directly applicable.
But on the subject with DHT, as with any androgen there would be dose dependent inhibition. I’m not familiar with what dose would average, for example, 50% inhibition.
With regard to Proviron itself, which it seemed to me was your real interest, unfortunately there is no study that can be found via Pubmed on this. I came upon it years ago (at least 12 years I would think) by chance while reading through the older literature. I suppose it could be found again by Chem Abstract search but that would be a pain and I can’t do it from here.
At any rate, there was only one dose studied – 50 mg/day – and LH suppression was fairly substantial. I certainly don’t recall the figure, but in the neighborhood of near 50%.
And though people seem to tend to find that contrary to expectation, it really shouldn’t be so surprising that an androgen steroid would be substantially suppressive at 50 mg/day.
Proviron does about nothing for muscle anabolism, and certainly doesn’t help restore HPTA function (on the contrary: it is somewhat suppressive) so it really doesn’t make sense for PCT. If some degree of support is needed or desired, I’d rather – on levels of injectables dropping to something commensurate with say the 100 mg/week level – maintain that level with ongoing small injections combined with a SERM than use the Proviron.
If nothing else, while that added small amount of testosterone would be comparably suppressive as the Proviron, at least it is doing something for muscle, which Proviron really does not, or not in any much-noticeable way anyway.
EDIT: I also now noticed your final question in the original post. There is no level of Proviron that corresponds to normal physiological. Even aside from the fact that normal levels of mesterolone are zero, there is no level of mesterolone that could be considered to directly correspond to a given level of either T or DHT due to its rather peculiar behavior of being a poor muscle anabolic while having good CNS effect.