[gpvos]

No, it's not, because it's not just a multiplication but also an addition. If I would live in the US for some time maybe I would develop a knack for it, but for now it's too much effort for something that only a few places use.

[jedberg]

I don't use multiplication or division. I just know 0=32, 10=50, 20=68, 22=72(aka room temp), 30=86, and 40=104 (aka really hot). I just estimate from there.

[zargon]

As a continental climate dweller I also know 0f = -18c, -20f = -30c, -40f = -40c.