If from the record you kept of your jobs in 2018 you can calculate the mileage accurately, then that would substantiate your business mileage. You could establish the total mileage on your car by comparing the odometer readings between two separate dates during the year, from repair bills or oil changes, etc. There is no one correct way to substantiate your mileage, but you should be able to accurately compute your deduction based on records. See IRS Publication 463 for more information.