I'll try to be polite,
Our planning system sounds similar to the one Galaxy Flyer describes. It compares the following three cases and ensures that we always have fuel to divert to the nearest available and suitable alternate in any of the following three cases:
1. Total pressurisation failure, followed by 2 Eng div at 15000'.
2. Engine failure and pressurisation failure, followed by SE div at 15000'
3. Engine fail, drift down, and single engine div.
It's very rare that we see extra fuel added to fill any shortfall at the critical point, that usually happens if you're heading West out of Europe over the mid Atlantic to, e.g. the Caribbean.....BWTF do I know....
It a catch 22 to ask an airline pilot how much the fuel burn of a 737 is at 12500 feet, if after 2.5 hours of flying at alt, he has the reserves to continue or get out the life rafts.
?????