An idea from WikiPortal, you can calculate NaturalPortals without taking the artificially-constructed forward and reverse indices into account. The argument is that the artificial portals taint the calculation. Detecting what constitutes a forward or reverse index is another problem. You can either do this functionally, say by excluding pages that are adjacent to 10% of the site, or you can do this editorially by flagging the pages like we have done here with CategoryHomePage. I've had to escape the link because pages that link to CategoryHomePage are excluded from certain IndexingSchemes.