When BF talks about established margin it's referring to the articles that may precede an entry word.
Your entry word begins in cell 1; subentries all begin in cell 3. The translation for the subentry follows on the same line as the subentry (separated by a colon). 21.9.2e is only used when there are article that precede an entry word. Follow examples 21-36 and 21-37.
The changing of the margin from page to page just needs to be handled logically...and in a way that makes it clear to the reader that they are looking at entries or subentries.