Oh, I see.
Step 1: regardless of C or F, add 40
Step 2: if converting to C, multiply by 5/9 (because C scale doesn’t go up as fast). If converting to F, multiply by 9/5 (because F scale goes up faster)
Step 3: regardless of C or F, subtract 40.
The nice thing about is that you just need to remember 40 and that the C scale isn’t as steep at the F scale.