In the book The Design of Everyday Things, Don Norman talks about what he calls "Good Enough Arithmetics", which is a way of calculating results in your head when precision isn't important. The approximate answer is often good enough.
I gathered a few notes on my website as a reminder, and plan to add more as I find them :neofox_book:
@rek I also enjoyed the book.
@rek some rough conversions I use pretty frequently:
1m = 39โ
6mm = 1/4โ
1m/s = 2kts
60mph = 100km/h
1 day = 100,000 seconds
Fun with ASCII ๐๏ธ ๐ ๐ค