[frutiger]

This is not an approximation. The conversion formula is:

C = 5/9 * (F - 32)

So GP's table is exact.

[tomger]

The article refers to C = 2 * (F - 30) as an approximation

[frutiger]

Ah, I had mistaken your reply, and assumed it was to /u/u801e and not the linked article.