Can we really 'discover' 85% of mammalian viruses?
For some statistical models, see e.g. The Mathematics of Biodiversity (Part 1) by John Baez.

However, in terms of new species, such statistical methods may not suffice - usually also our tools for discovering change. So it might be more accurate to extrapolate number of some kind species we know as a function of time.

For extrapolations (though not for viruses) see e.g.:


Camilo Mora et al., How Many Species Are There on Earth and in the Ocean?, PLOS Biology (2011).


In any case, for viruses (and many other fast-multiplying asexual creatures) defining number of species can be anywhere from tricky to impossible (for fundamental reasons, not only - practical). At best you can try to estimate their time to the last common ancestor. See also links in this discussion on Azimuth.