What's in a formulae???

What's in a formulae???
Well lotss.... This one made me feel uncomfortable for 3 full days..

As I boarded the flight to San Francisco from Heathrow, I was worried about how cold it is going to be when I landed..

So I kind of fell back on a formulae that I must have learnt at least 10 to 12 years ago. The formulae was for the conversion of Temperature from Farenheit to Celcius..Seemed to me like

c/5=(f-32)/8

But somehow I was feeling like something was wrong. Seemed allright but something was wrong.. Did not have access to the internet for the next 2 days.. And finally when I did google it out, the error was marginal. It is more like this

c/5=(f-32)/9

May be just a 0.1 difference in the multiplication factor, but all the same, I lost peace of my mind for 2 days over it...