Keith, like your formulae, however I think you are inadvertently being a little obscure when you are using OAT in your ISA calculations - you should use ISA TEMP until you introduce the deviation lower down.
Yes, the ISA defintition assumes a sea level temp of 15°C, reducing by 1.98°C up to 36,000 ft, and then remaining constant at -56.5°C.
(The ISA also assumes that the air mass has no moisture, water vapour, dust or other "impurities" and that it confirms perfectly with the gas laws.)
Just as the sea level temperature is usually different from day to day, so is the lapse rate, the humidity and every other assumption about the ISA - including the tropopause height, and the temperature of the tropopause. I have personally seen a temperature indication of -71°C (in winter about 35°S at FL450).
In fact, as you go even higher, the air temperature starts to increase, due to chemical reactions in the mesosphere.
[ 08 November 2001: Message edited by: Checkboard ]