Approach
Bottom-up
Start with customers, usage, and spend.
When to use
Use this as the primary path because it matches the data a candidate can estimate quickly.
Start from locations, accounts, or transactions, then multiply by capacity, penetration, and price.
Approach
Top-down cross-check
Start broad, then narrow by geography, segment, and capture.
When to use
Use this as a sanity check to catch impossible order-of-magnitude answers.
Cross-check against population, household, or company counts.