I found this graph on the blog site. I am not sure of the source nor the accuracy, but it does somewhat address your question and explains much of what I know about the drug fnasteride.  When Merck did the initial clinical trials, they were trying to determine what the best and most effective dose of finastride (Porpecia) would be and then, of course, what they would recommend. They found that doses greater than 1 mg, on average, did not increase the value of the drug on hair loss. They also found that when half the dose was used, that the value was 80% as good as the full dose. Going down in dose as well to 0.25 mgs would have half of the value, so I think that this curve is somewhat correct. At least it does explain why some people can get an impact on a lower dose.  But Merck recommended 1 mg because it produced the best effect on MOST people.

finasteride inhibition curve