"Spell it the way you pronounce it" seems pretty simple and straightforward, when it come to what Fowler called "possessive puzzles".

Interestingly enough, the form where you leave off the 's when the word ends in s already is apparently the older one. "Formerly customary" is how Fowler put it, going on to point out (back in 1926) that it wasn't customary any more, except in a few special cases.

So, if Americans find themselves impelled to do it that way in writing, even when they wouldn't in speaking, that would be another example of a frequent national tendency to stick with the older way of doing things. (That's not a criticism, just an observation.)