One-point calibration, as you suggest, really isn't calibration. Your cooks' thermometers will be great at testing temperatures near freezing, but probably not so good at measuring higher temperatures, like safe serving temperatures. It all depends on the probe/meter. I recall when this question has come up before, the Apex probe won't read accurately at all around freezing. So "calibrating" it there will do more harm than good.
It's really no different than calibrating a refractometer - if you're calibrating using a single point, you want that single point as close as possible to what you're trying to measure. In our case, you'd want to set the temp probe to something that you *know* is around 80 degrees. Pretty tough.
If you're determined to have an exact reading, you can buy a thermometer that is pre-calibrated to NIST standards. Expect to pay a bit for it. Then use that as your "actual" temperature and set the others based off that. For me, that *actual* temperature isn't as important as the consistency. If I'm off by a degree or two, it's not the end of the world. My fish and corals are used to 79 degrees... whatever that may actually be!
