It depends on what you want to have. If your goal is to retrieve all genres anyone ever tagged this with, set it very low. If you want to filter out outliers a higher value (something between 60-90 %) is more appropriate. This is still subjective crowd sourced data, with the idea that given enough people tagging the most relevant genres will get visible, forming kind of a crowd consensus. The idea of the min genre usage value is to filter out those genres that got significantly less votes than the highest voted genre. If a hundred people tagged something as “Neoclassical metal” and one put in “Hopepunk” (just picking some stuff from recent discussions) I would give some trust that “Neoclassical metal” is a proper classification of this piece of music while “Hopepunk” isn’t (even if I have no idea what either of them actually is). But if 90 people had tagged it “Hopepunk” then probably both genres describe this music well.
I had written a bit how this works at Picard 2.x and genres. Good practices? Advice on plugins? - #12 by outsidecontext
But I agree that we could set this lower to maybe 70% now that we default to actual genres.
You can already do this, and actually it is the default on new install. Just disable “Use folksonomy tags as genres”.
The genres from recording, release and release group are combined (including their votes). If you have enabled artist genres the genres of the artist get also merged in.
Yes, set test.musicbrainz.org as the server address in Options > General.