Yeah, found something, from http://www.davidappleyard.com/english/spelling.htm
When a suffix beginning with a vowel is added to a stressed syllable
ending in a single consonant preceded by a single vowel, the consonant
is usually doubled, as in:
control > controllable
In British English, a final letter l following a single vowel is doubled even if the syllable is unstressed: travel > travelled.
American English not only adheres to the usual rule requiring the final syllable to be stressed before doubling, it doubles the final l in all forms of the verb, thereby eliminating this particular spelling headache altogether:
AmE enroll > enrolled
and fulfill > fulfilled;
BrE enrol > enrolled
and fulfil > fulfilled.
stop > stopped
admit > admitted
In the following cases the stress in the final syllable is secondary:
kidnap > kidnapped
program > programmed
Consult a dictionary before doubling a final s to form noun plurals, especially in monosyllabic words:
gas > gases;
bus > buses (but AmE busses).
A final z is always doubled:
fez > fezzes; quiz > quizzes.
In words of more than one syllable, both British and American English follow the usual stress rule when adding -es to form the third person singular of the present tense:
focus > focuses;
nonplus > nonplusses.
In British English (as in the case involving a final l above), a stressed syllable is not a prerequisite for doubling the s before -ed and -ing to form past tenses and gerunds.
So BrE grants you the option of either focussed or focused
and focussing or focusing.
AmE, on the other hand, prefers the latter variants (focused and focusing), which follow the general rule about stress.
Instead of doubling a final consonant c, which only occurs in unstressed syllables, it becomes ck before the addition of a suffix:
traffic > trafficking;
frolic > frolicking.
The consonants h, w, x and y are never doubled (e.g. affix > affixing), and neither are silent consonants found in words of foreign origin:
crochet > crocheting;
ricochet > ricocheting.
big > bigger
begin > beginning
refer > referring
red > reddish
Boy, lots of rules, eh?!