That's -40f for those south of the border.

No need to thank me for doing all that math. On a serious note, some actually helpful points are:

0c = 32f (probably the most well known one)

10c = 50f

20c = 68f

30c = 86f

40c = 104f

Aside that, I figure I'll be cooking and therefore using a nice enough thermometer to have both scales. Otherwise things have gone terribly, terribly wrong, and I have much larger problems to worry about than conversions.