Help one hour single engine 737 calculation

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Here's a question for you. (Especially B737's)

If your destination happens to also be your One Engine Inoperative or Depressurisation Alternate, how are you going to demonstrate during the flight planning stage that you have enough fuel to arrive with the required reserves in the tank?

This question assumes that you want to dispatch with the minimum "Normal" fuel required. How do you know this figure doesn't require a "contingency fuel" top up??