I'd like to point out that metric is based off completely arbitrary benchmarks as well.
All metric distance measurements are based off the meter, which is based off a flawed measurement of the earth's diameter from the French Revolution period (was supposed to be 40,000 km for the diameter of the earth when in reality it's 40,075.017 km) . Then in 1983, they decided a meter is the distance light travels in vacuum in 1 ⁄ 299,792,458th of a second, which is a pretty damn illogical and arbitrary number, I think, especially since time measurements are based off an inaccurate measurement of solar and lunar cycles.
The gram is based off the meter, so the whole metric weight system is based off an arbitrary value as well.
Celcius is based on the freezing point of pure water at sea level, which makes less sense the Farheinheit which is based on the average range of tempeture range of Europe for describing weather and less sense then Kelvin for scientific use. So with celcius, you have a smaller whole number range to describe the weather and it tends to use a lot of negative numbers and fractions as a result. A more logical and scientific measurement system would be Kelvin, which is based on the temperature of absolute zero. No negative values with that system.
Anyways, a quick way to roughly convert Celcius to Farheinheit in your head is to double the number and add 30, So 20C is 70F, 10C is 50F, ect its gets you pretty close if you have to decipher weather forecasts in Europe or whatever (and isn't a negative number).