The Minimum Purchase setting defines the amount required before the Tax Formula applies. If the sale amount is less than the Minimum Purchase amount, the Tax Formula is ignored. To ensure a Formula applies to all sales, leave the Minimum Purchase field set to zero.
Note: The Minimum Purchase dollar amount is not subtracted from the Taxable amount when a sale exceeds the Minimum Purchase. Tax is calculated on the entire Taxable amount and not just on the amount over the Minimum Purchase (unless the Subtract Minimum Purchase Amount setting is enabled (as it is by default).
See:
Tax Tables - General Information
Configuration Manager Contents