Thanks Ron. I think the formula in that adafruit blog is essentially the same as the one I posted from atmel, except the stray cap is not doubled in atmel formula. Atmel says that is only needed for low-frequency crystals though. I wonder why they only provide this formula for low-freq.

If I calculate with adafruit formula, assuming a 5pF stray cap, C1,C2=2*18-2*5=26pF.

If I calculate with atmel formula, C1,C2=2*18-5=31pF.

Either way 22pF seems insufficient. I have to have a lot of stray to make that work.

I'm relying on the crystal to do USB full-speed device so I can't be to off.

If I had CL=12, then C1,C2= 14pF or 19pF. I guess 22pF might be alright when there is less stray.