Atmel's AT42QT2160 uses a 28-lead QFN/MLF package. The datasheet for this part doesn't have package dimensions! So I searched around and found that their ATmega168PA/328P also comes in the same package. But that datasheet has dimensions for for a 20-pin QFN (which the 168P does not even come in). Thanks Atmel, you are a ton of help today.
So after wasting time searching, I'm stuck calculating what I hope are the 28-pin package dimensions, and if I'm wrong, eventually I'll be wasting a LOT of time redesigning my board.