I find the conversion between the two easy enough to do it my head.

This isn't exact but is close enough for conversations and 99% of my needs.

(Temp in F - 30) / 2

Examples

70F:
70F - 30 = 40
40 / 2 = 20C

10F:
10F - 30 = -20
-20 / 2 = -10

The actual number is 21 / -12 but this is close enough for me 99.9% of the time