Let's use FL 310 as an example.
So we take 31 and multiply it by 2 and get 62. That 62 will be subtracted from 15 to get -47. My charts say -45, which is close enough for me for me to figure out cruise performance. That was easy!
But here is one more thing that's bugging me. Engine performance data is based on TAT, but some of our planes give SAT, my Check Engineer says just subtract between 22 and 25 degrees from SAT to get TAT. My CR-2 Manual dances all around the subject *WITHOUT* showing how to convert them.