With four possible algorithms now, it would be nice if these options could be consolidated into a single, user-orderable, priority list (kind of like the formats list in the dialog).  Calibre would then start at the top, try one method if it failed (didn't produce a page mapping) try the next, and so forth.  As is, pagecount will always override if there is a non-zero value in the custom column and pagebreak and accurate are mutually exclusive.
Barring that, I see two reasonable priority lists:
1) optimized for accuracy: pagebreak, pagecount, accurate, fast
2) optimized for speed: pagecount, fast (do not use pagebreak or accurate)