There are two possible sections in TaxCalc where you can enter the mileage figure.
Your Work > Employment > Fringe Benefits > Vouchers, credit cards and excess mileage allowance > Mileage Wizard.
OR
Your Work > Expenses > Business travel and subsistence expenses > Mileage Wizard.
Employment > Box 12 > Click on the wizard icon to access mileage wizard.
OR
Employment > Box 17 > Click on the wizard icon to access mileage wizard.
If C (Total approved mileage allowance payments minus Allowance received from employer) is positive, the result goes in box 17 and decreases your tax liability.
If C is negative, the result goes in box 12 and increases your tax liability. If any data is entered in the last field on the wizard it will be added to any figure that goes into box 17.