Forgot to mention something very important:
Standard tropopause temperature is -56.5 at 36,200 feet. This is a standard atmosphere and it does not mean that if your OAT shows -56.5 degrees, that you are at the tropopause. At southerns latitudes where the the Tropopause is much higher you will find much colder temperatures than that for the trop. Near the equator, the tropopause is at its coldest. Let's say today the tropopause where your airplane actually flies is 45,000 feet, trop. temperature could be close to -75 degrees (45x2-15). You see, we're very far from our -56 degrees, which is only a standard, just like 15 and 1.98 degrees are.
So using your QRH, when above 36,000, whether you're at 37,000 or 45,000 feet, with an OAT of -66 degrees, you're ISA-10, because it was established long time ago that in a standard atmosphere, the temperature does not change at all above 36,200 feet. So I use my formula (altx2-15) up to 36,000 feet, above I use -55 degrees for quick reference.
Cheers!!!