I think I've posted this before, but a neat way to get ballpark figures is to remember that at two points they are mirror images:

28 C = 82 F
16 C = 61 F

I find it easy to remember because you only need to remember two points for ballpark estimates. Both are also good reference points when you try to "imagine" what they're like - 16C/61F is a bit below comfortable room temperature, and 28C/82F is a pretty warm summer's day where I live.