The quantity you are estimating
Decide whether you are estimating people, transactions, units or revenue. Define the geography, customer group and time period. Annual bicycle sales are different from the number of bicycles people already own. Keep the definition visible throughout your calculation.
The calculation comes first
For annual new electric-car demand, one possible calculation is households multiplied by the share owning a car, the annual replacement rate and the electric share of replacement purchases. State the simplifying assumption that a car-owning household has one relevant car. A different assumption would require a different calculation.
People and purchase frequency
Population estimates usually need a participation assumption. Transaction estimates also need frequency. If 8,800 students take two bus trips per study day, four days per week for 30 weeks, the annual total is 2,112,000 trips. Each step has a unit, and trips per year are the output.
Uncertain assumptions
Rounded assumptions are easier to work with, provided you explain them. If purchase frequency is uncertain, calculate a lower and upper estimate with the other inputs held constant. You can then see how much that assumption changes the result. Decimal precision adds little when the inputs are rough estimates.
Checking the estimate
Divide the total back into a quantity that is easier to judge. A city estimate of 27,000 gym members among 90,000 adults means 30% membership. Check whether that matches your starting assumption. For revenue, divide annual revenue by customers and compare the result with the price and purchase frequency you used.
Someone following your calculation should be able to see how each assumption affects the estimate.