Which amount is used in calculating property tax when Juan's property is assessed at $387,000?

The amount used in calculating property tax is the assessed value of the property, which in this case is $387,000. Property taxes are typically determined based on the assessed value, as this figure reflects what the local government has determined the market value of the property to be for taxation purposes.

Therefore, when Juan's property is assessed at $387,000, this is the value that will be used to calculate his property tax owed. Other choices presented do not represent the assessed value of his property and thus would not be used in this calculation.