This won't give you the specifics on how to adjust your thermometer, but it is a pretty cool poster: http://www.foodsafe.ca/downloadfiles/FSFoodservices12-ThermoCalibr.pdf
BTW, when I googled "weber thermometer calibration" your question on this came up on the bottom of the first google page.
Yep, mine is so far off that it is worthless. Buy a digital and stick in one of the lid vents, when my digital reads 330 the weber lid thermo is up to about 210 or so.