well the ratio check is skipped if uploaded=0 (line 412). I've tested these possible uses:

clean_transmission: yes
(clear all finished torrents, even if they are still seeding)

clean_transmission:
  olderthan: 1 days
(clear fully downloaded torrents at least 1 day old, regardless of upload ratio)

clean_transmission:
  uploaded: 0.5
(clear finished torrents only if they have been uploaded up to 50%)

clean_transmission:
  olderthan: 1 days
  uploaded: 0.5
(clear finished torrents if they are older than 1 day or if they have been uploaded up to 50%)

I can change names and/or defaults btw.