Macie Automated sensitive data discovery Cost/Scan Threshold

You put in the total amount you have in storage and Macie will automatically determine the ratio that will actually be scanned. From the Calculator - "Macie picks small samples of objects from your S3 for inspection. This calculator provides an approximation of the data picked." You don't have to do the math the calculator does it for you. The below is also from the Pricing FAQs, the trial will allow you to estimate. -

Q: How do I estimate the cost of the initial enablement of Macie on my account?

A: You can enable the service and take advantage of the 30-day free trial. During that period, you can access a usage tab in the Macie console that will estimate your usage for S3 bucket-level inventory and an evaluation for security and access controls. The console will also estimate your automated data discovery, which includes the data inspected for sensitive data discovery and the objects monitored in your S3 storage, before transitioning to paid usage. However, your estimated usage during free-trial can be lower for accounts which have more than 150TB data. Please contact AWS Support for assistance with a more accurate estimate. https://aws.amazon.com/macie/pricing/