Define who could actually use the service
Start with the customer unit and catchment. An individual product may need people of particular ages; a shared home service may need households; a business service may need establishments. Define the place as carefully as the audience. A state total or urban agglomeration may cover a much wider area than a branch, delivery route or neighbourhood service.
Use a census count for a historical baseline or a labelled government projection for a planning scenario. Keep the boundary and date next to the input instead of treating every available population total as interchangeable.
Add evidence that matches the question
Consumption surveys can inform spending categories and rural or urban patterns. Labour surveys add participation and employment context. Migration and tourism sources describe different kinds of mobility and activity. These sources help test a market hypothesis, but they do not directly reveal the number of customers willing to buy your offer.
Choose compatible years, units and populations. Avoid assigning a national spending average to every local household or treating repeat tourist visits as unique potential customers.
Keep the estimate reviewable
Build a scenario from the addressable population, relevant eligibility or need, plausible participation and spending assumptions. Separate official inputs from your own assumptions and try a range where evidence is weak. The market-sizing tool helps organise that reasoning; the result remains a scenario.
Finish with the local questions that would most change the answer: actual reach, alternatives, category use, willingness to pay and seasonal demand. This makes a demographic starting point useful for a decision.