NI Adjustment for Mileage Paid under 45p when also receive Car Allowance

For companies that pay car allowance and mileage at a lower rate of 45p employers and employees should not be charged NI on that following Rouke & Wilcott case. Employees can claim their tax back on their self assessment however that doesn't release the NI that needs to come back through the employer. An input box that carried out this adjustment automatically would be useful i.e. adjust the car allowance down by the value difference between 45p per mile and the amount actually paid, then put back through the system with tax only applied/ no NI applicable.