How do I calculate a realistic FinOps savings baseline before a cost optimization initiative?

Use a trailing 90-day average of actual spend (not a single month, which can be skewed by one-off events) as your baseline, and separate committed-use discounts already in place from the gross list-price spend.

Common mistake

Reporting savings against list price when reserved instances or committed-use discounts are already active double-counts savings that were already locked in before the initiative started.