You should set
2 stars to 0 'ppd' tracks and
other stars for greater 'ppd' values. Notice
that standard rating for never played files deals with
'last played' tag - not with 'play count'/'skip count' (i.e. 'never palyed' tracks should have 'Unknown' 'last played' tag value). See
auto-rating values in screenshot: 2, 4, 5. For tracks with greater than 0 'ppd' value
you can set up percentages of tracks which must have certain rating (but keep in mind that its not always possible to satisfy your conditions for splitting library into groups with different rating (for instance if
all tracks in your library have the same 'ppd' value then all tracks will be assigned highest possible rating). In case of usage of percentage you should calculate thresholds manually or every time on startup.
EDIT: I've changed 'auto rate' command behavior for the next update so that if 'play count' is 0 then track is considered to be never played even if 'last played' tag is not equal to 'Unknown'.