Possessive case for a certain proper noun - ss apostrophe [duplicate]

This is a style issue, not a matter of objective correctness or incorrectness. Having said that, I note the advice that various style manuals offer.

From Words Into Type (1974):

Proper names. The possessive form of almost all proper names is formed by adding apostrophe and s to a singular or apostrophe alone to a plural.

By this style rule, you would express the plural of Ross as Ross's.

From The New York Time Manual of Style and Usage (1999):

possessives. Ordinarily form a possessive by adding 's to a singular noun (the boy's boots; the girl's coat), even if the noun already ends in an s (The Times's article). If the word ends in two sibilant sounds (ch, j, s, sh or z) separated only by a vowel sound, drop the s after the apostrophe (Kansas' climate; the sizes' range). But keep the s after the apostrophe when a name ends in a silent sibilant letter (Arkansas's; Malraux's).

Here the possessive of Ross would be Ross's but the possessive of Biswas would be Biswas'.

From The Chicago Manual of Style, Fifteenth Edition (2003):

7.18 Proper names, letters, and numbers. The general rule ["Form the possessive singular of nouns by adding 's"] covers most proper nouns, including names ending in s, x, or z, in both their singular and plural forms, as well as letters and numbers. {Relevant example:] Strauss's Vienna

...

7.23 An alternative practice. Those uncomfortable with the rules, exceptions, and options outlined above may prefer the system, formerly more common, of simply omitting the possessive on all words ending in s—hence "Dylan Thomas' poetry," "Maria Callas' singing," and "that business' main concern." Though easy to apply that usage disregards pronunciation and thus seems unnatural to many.

Here, by 7.18 you'd use Ross's, but by 7.23 you'd use Ross'.

I haven't found any instances where a stylebook instruct its followers to treat the possessive of a singular proper name differently if it ends in a double s (as with Ross) than if it ends in a single s (as with Barnes).