In the same vein, converting pounds to kilos is "divide by 2, subtract 10%".
no way just discovered this lmao i now only need a similar thing for fahrenheit to c/viceversa, i'm sure it exists. however, i'm so lazy i will not search for that information now
For Celsius to Fahrenheit, just double it and add 30. It gets you pretty close for the temperature range that humans live in.
That's how I do many kinds of percentages... like 72% is 50% and then 10% twice and then a couple of 1%s