Centigrade vs Fahrenheit cheat sheet

Celsius = 5/9 (F - 32)

Fahrenheit = (9/5 C) + 32

(2C° - 0.1(2C°)) + 32

Double C and add 32 becomes less accurate as the temperatures rise. It is exactly right at 0 C, off by 8 at 40 C, and off by 20 at 100 C.

The doubling is off by 10%. 2 is 10/5, but the correct formula is 9/5, so the right factor is nine/tenths below the fudge factor.

Double C, subtract roughly 10% of that product from itself, then add 32 improves the result.