Based on what I learned in English, you add ('s) for possessive words that:
1) don't end in (s): Micheal's, Ronald's, The dutch's, The fleet's
2) are mono-syllabic that end in either (x) or (s): Rex's, Hans's, Ross's
3) are multi-syllabic that end in (x): Xerex's, Pyrex's

You don't add ('s) for words that are:
1) multi-syllabic and end in (s): Jesus', Reyes', Laos', Cyprus'

Hope that helps :)