Interesting strategy. I long ago memorized 20C = 68F and then adjust in increments of 5/9, which works well enough for the range of temperatures I consider habitable. I've gotten so good at this I once translated on the fly between some fellow American and European runners during a marathon. :-)

"When it's springtime in Alaska, it's forty below". That's all I have managed to memorize about temperature conversion so far in my life.