I have this Celsius Fahrenheit thing worked out perfectly (if slightly indefensibly).
http://inspire.ospi.wednet.edu:8001/curric/weather/fahrcels.html

0 degrees sounds pretty cold but 32 doesn't do anything for me so I stick to Celsius for cold weather.

In the Summer I'm much more impressed by the weather being in the 90s than the 30s, so I stick to Fahrenheit.

For everything else in-between, below 15 - Centigrade, above 60 - Fahrenheit, simple really!

The sad thing is that I know I'm not the only one who thinks the same way. Our weather forecasts often show both, so we don't even need to do the sums.

It's a bit like buying 48 inch wide fabric by the metre or 5 metre wide carpet by the yard - we do that too!

When calculating other currencies abroad, my speciality is to get the 1-9 numbers right but to be way off the mark with the 0000s - sometimes I refuse to buy something because I think it is outrageously expensive (in Turkey recently 1,000,000 Turkish Lira = £1) or I comment on how cheap things are and find out I'm out by a factor of 10.

Funny things, numbers!