The standard formula is to multipy your inseam by .67. So:

82 x .67 = 54.94

Based on this and assuming that your inseam is measured correctly, a 55 cm frame would be ideal, but I since they don't come in that size the 54 cm should work very well also. I'd tend to round down to the 54 rather than go up to the 56 size.

Also make sure that you're measuring the inseam correctly before making a commitment. The proper way to do it is on this site.

Cervelo sizes their tri bikes to road sizes, so this will work fine.

http://www.coloradocyclist.com/bikefit/