I learned an easier method years ago. Since both Celsius and Fahrenheit are the same at -40 degrees an easy conversion is to just add 40 degrees to the temp. you are converting and then if you want to go to fahrenheit you need more numbers so multiply by 1.8 and then subtract the 40 degrees back off. If you are going from Fahrenheit to Celsius you divide to get less numbers, then again subtract the extra 40 back off.
Either way at the start add 40 and at the end subtract 40. The trick is figuring whether in the middle step to multiply or divide by 1.8 ..
As an example 100C = 212F the boiling point of water.
Changing to Fahrenheit the degrees are closer together and you need more of them so multiply not divide.