for what it's worth: add 9 degrees F (or 18 if it's easier to remember) for every 5 degrees C (10 C, easier), and peg 32F to 0C. You get:

-40F=-40C

-22F=-30C

-4F =-20C

14F =-10C

32F = 0C

50F = 10C

68F = 20C

86F = 30C

104F= 40C

and then approximate in between from there. It's quick enough for me now that I skip the 2x+30.