TIL: In American English, the general rule for whether to double the final consonant in "X-ing" is whether the last syllable is stressed.

It's more complicated -- for unstressed endings, based on a list of consonants that varies (for p & l) between UK/US English.

I have a thorough reference for this at home, with examples... although I'm not sure which 1 of 3 books it's in...

Though I recall the canonical US/UK differences are travelling and worshipping (UK) vs. traveling and worshiping (US)

So Webster's Third New International Dictionary isn't the source I remembered