[Bwana]

You can go with your "best guess," but the way to know for sure is get yourself a good commercial grade meat thermometer, preferably two. The internet is a decent guide, or starting point, but as you said, there are several different instructions for the same size bird. You have to take into consideration altitude, the variations in temperature deviation in different ovens, etc.

I own 10 TEL-TRU thermometers that I use for smoking. When I smoke, I like to really dial things in. I will also use two of them for a turkey, or a prime rib. I use two so I can make sure there isn't any kind of issue going on within the meat. You can get a cheap Wal-Mart thermometer, but you're stuck with a cheap thermometer. They may or may not work. When I'm cooking a $100+ piece of prime rib, or a decent Turkey, that doesn't cut it.

Like I said, step up to a TEL-TRU or two and it will be the best cooking investment you can have. You can find deals on-line, by taking some time to look around.

http://www.teltru.com/s-44-food-service.aspx