Wiktionary says that the difference between "targetting" and "targeting" is that the first one is a British spelling and the second one is American. Meanwhile, Oxford Dictionaries says that "targetting" is a misspelling of "targeting". Which of them is correct? Is it the case that if I would write "focussing" rather than "focusing", then I should write "targetting" instead of "targeting"?
Here is an Ngram chart that matches targetting (blue line) and targetted (red line) against targeting (green line) and targeted (yellow line) in British English publications over the years 1950 through 2000:
The most striking thing about the chart (aside from the low frequency of all of these forms as late as 1970) is the continued low frequency of targetting and targetted.
The Oxford Style Manual (2003), at 11.4.2, lists the ways in which the normal U.S. English treatment of words with suffixes beginning with a vowel differs from the corresponding normal British treatment:
Before a suffix beginning with a vowel, the final -l is doubled [in U.S. English] only where the last syllable is stressed: labeled, jeweler, counselor, traveling, marvelous, quarreled, rivaled; but note fulfill, skillful,thralldom; occurred but worshiped, kidnaped. Before a suffix beginning with a vowel, the final -e is often omitted where in British usage it is retained, as in milage and salable.
This is mostly accurate, except that (1) mileage is the standard U.S. spelling, (2) kidnapped is the more common U.S. spelling, and (3) labelled, travelling, and worshipped are common U.S. variant spellings. But what is most noteworthy here is that the style manual doesn't mention the absence of -t doubling in U.S. spelling at all. That's because, on both sides of the Atlantic, targeting, pivoting, parroting, etc., are normally spelled without -t doubling before a suffix beginning with a vowel.
As for focusing versus focussing, Merriam-Webster's reports that the doubled -s form is a common variant in U.S. English, so the premise that focusing is the only acceptable form in U.S. English is much shakier than you might suppose.
One further interesting case is programed versus programmed: Merriam-Webster's Collegiate Dictionaries through the Sixth Edition (which was still being published in 1963) gave programed as the predominant U.S. spelling; but the Seventh Collegiate (1963) and all subsequent editions have listed programmed first.
My understanding of the use of a double consonant when ending a word in 'ing' or 'ed' is when the preceding vowel is either 'long' or 'short'. a 'long' vowel is one that sounds the same as the name of that letter. I assume the term 'stressed' means the same as my definition of 'short'. That is, the phonetic sounds that we (the older generation, at least) learnt before we could read. Words such as cap, set, tip, top and cup all use a short vowel. If you were to add 'ing' to these words, they would all have a double consonant. If there were a 'long' vowel (or, indeed a double vowel), then the following consonant would remain single. Words such as 'follow' and 'snow' are a couple of examples. If the vowel and consonant combination is followed by an 'e', the 'e' is dropped when adding an 'ing'. Of course, I am sure there are some exceptions to this rule, as only the English language can do. The rule I was taught in early primary school is, "Short vowel, double consonant - long vowel, single consonant'.