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One of my (older) English reference books says use whichever one you prefer, and whatever reads better (I would prefer Ross').
However, the Cambridge Guide to Australian English Usage says that the possessive form of proper nouns ending in -s has led to "great diversity of opinion and a variety of practices" (ie a lot of disagreements!).
The early convention was to exempt all proper nouns ending with S from the usual rule (add apostrophe S), and just add the apostrophe, and no S (hence Ross', Jones', Jesus', Xerxes', or Menzies'). Now it seems that the following variations are in use:
Rule 1) Usual rule applies (ie add apostrophe S at the end), but not for literary, classical, or religious names (Ross's, Jones's, Menzies's, Dickens', Jesus', Xerxes'). In this example, Ross and Jones and Menzies don't fall into the category of 'literary, classical, or religious names', so they get the 'apostrophe S' treatment, but the other names don't.
Rule 2) Usual rule applies, but not for names of two or more syllables (Ross's, Jones's, Menzies', Dickens', Jesus', Xerxes'). In this example, Ross and Jones are short names, one syllable long, so they get the apostrophe without the S, but the other names, which are all longer, get the apostrophe with the S.
Rule 3) Usual rule applies, but not for names whose last syllable sounds like 'eez' (Ross's, Jones's, Dickens's, Menzies', Jesus's, Xerxes'). In this example, the only names who end with the 'eez' sound are Menzies and Xerxes. The other names don't end with this sound, so they get the apostrophe S.
Rule 4) Exempt names whose possessive form is pronounced with the same number of syllables as the plain form. The application of this rule depends on personal pronunciation of the name.
The easiest way to get conformity in this complicated system of arbitrary 'rules' is to just apply the apostrophe S in all cases across the board (which is what the Australian and USA government style manuals currently recommend) — with some exceptions for literary, classical, or religious works, occasionally, where the older tradition of just using the apostrophe, and no S, is more usual.
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