[Elecraft] solder iron correct temp in C for K3

[Monty Shultes]

Another trick, you can do C->F in your head:

Double the centigrade temperature, subtract 10% from the product, and add 32 to get Fahrenheit.
   10 degrees C =  (2*10-2) + 32 = 50 degrees F

F->C is not so slick, but works:
Subtract 32 from the Fahrenheit temperature, add 11% of the result to itself and divide by 2
   50 degrees F =((50-32) + 1.98)/2 = 10 degrees C.