What percentage of hentai manga artists are female?
The answer is "we do not know".

Now, as others have pointed out, there is the issue of Mangaka using (multiple) pen names for their erotic stuff, but the other half of the story is that Mangaka like to stay anonymous even in a more general sense. Most of them don't like attention, which is why we don't even know who wrote, for example, Death Note. Could be a guy, could be a gal. 

For erotic Manga things just get more secretive.

If I had to guess, I would look at the number of erotic Manga aimed at guys (~22.500) and the number of erotic Manga aimed at gals (~2.300) to get a rough estimate of at least 1:10.