I've seen hacks that say to use a scale of 2x to convert between Celsius and Fahrenheit (with 30 Fahrenheit being added when converting from Celsius or subtracted when converting to Celsius). The problem that comes up with that conversion is its inaccuracy the father you get from 10 Celsius/50 Fahrenheit. Temperatures for ovens and freezers are inaccurate with this method and miscalculations can cause damage to things put into them.
I've used the conversion where you subtract 32 before multiplying when converting from Fahrenheit and multiply then add 32 when converting to Fahrenheit, but sometimes you forget a step and come up with the wrong temperatures.
To convert quickly and accurately between Celsius and Fahrenheit, add 40 (because -40 is the same temperature on both scales), multiply by 5/9 to convert from Fahrenheit to Celsius or 5/9 to convert from Celsius to Fahrenheit, and then subtract 40.
5(F+40)=9(C+40)
Note: If converting to Kelvin temperatures, add 233 instead of subtracting 40 at the end. If converting from Kelvin temperatures, subtract 233 instead of adding 40 at the beginning.