Miles and kilometers can be estimated by the Fibonacci Sequence. The conversion (mi -> km) is very nearly the golden ratio (1.61 and approximately 1.62, respectively, IIRC). For any number in the sequence taken as miles, the subsequent number is the distance in kilometers. Your way is probably quicker, but it’s a fun bit of information.

Room temperature is 21°C and every degree Celsius is worth about 2°F; that’s about all it takes.