I've seen the short answer to this from multiple sources as the following tables (in microns):
Detail rendering equivalency:
- FM 20 = 500 lpi
- FM 25 = 325 lpi
- FM 35 = 325 lpi
- FM 36 = 275 lpi
However the answer to your question is probably much more complex since there's other factors including paper and the consensus that stochastic printing generally has more dot gain, which will increase the size of the dots and has to be compensated for with dot gain compensation curves. There's also different ways of measuring the lpi for FM screens, the table above refers to how many dots are encountered on a 1/16˝ line, whereas there's also a lithographic method where the edges of dots are counted in a given area.
For more details and a general overview, see the Ed article above. For the two methods of measuring the lpi and the source of the tables above see this blog post (by a member of a forum on printing I frequent) and for a discussion of dot gain and other FM production issues see this blog post.