That was only about plural possessives though, wasn't it? Now we're talking about the possessives of
singular nouns ending in "s".
I must admit I don't really have a 'rule' for when to add an extra s and when just to leave an apostrophe. My practice varies with different words.
"Practice varies widely in for conscience' sake and for goodness' sake, and the use of an apostrophe in them must be regarded as optional" The New Fowler's Modern English Usage, ed. Burchfield, RW, 3rd edition, 1996, entry for "sake", p. 686.
I think that's talking about whether to write "goodness' sake" or "goodness sake", though, isn't it? Not "goodness's sake" (which just seems odd, given that in that phrase you don't pronounce the extra "s" at all).
