Hi Gary,
I asked the FDA why the recommended 180-185° when they, themselves, said that all bacteria was killed by 160°. She said that it tasted better. I asked her, "Says, who?" She said, "A committee." Cooking to 185 is why many folk have problems with dry turkey.
If you had your copy of "The Great American Barbecue & Grilling Manual" handy, you would have found the answer to that question and the method for calibrating thermometers. Check it out in the Barbecue Store at www.barbecuen.com
The way I calibrate mine, over here in McComb, is pour a little Tennessee whiskey over ice and stir with the thermometer probe. It should read around 32° then I put the tip in boiling water and it should read about 212° in Vicksburg.
Have fun,
Smoky