Article 58 of the Decree-Law mandates that the Executive Regulation will define the specific calculation method for recoverable input tax. This rule applies when input tax is paid on goods or services during a tax period that are utilised for a combination of purposes. These mixed uses include making taxable supplies that allow for recovery under Article 54, making supplies that do not permit recovery, and conducting activities outside the normal course of business. The article essentially delegates the technical apportionment and adjustment procedures to the Executive Regulation to ensure accurate tax accounting for mixed-use inputs.