That's a clever mental math trick. But Celsius and Centimeters never really get used alot in the U.S. except for healthcare, lab work, and some instruction manuals. So I try to relegate conversions to my handy cell phone apps and keep my personal mental processing power available for math and memory. Like you - I learned some mental math tricks like doing square roots manually and I'm constantly catching errors at checkout counters.
Now if the U.S. switches to the measurement standards of metric like the rest of the world - I'll have to get yet another math method engaged
Lets see -40C = -40 x 2 = -80 - 8 = -72 + 32 = -40 F And the result of the formula is . . . . nobody wants to be outside on a tractor at -40 "anything" LOL.