If your calculation involves deducting at AFR, your personal miles are calculated as a deduction, being multiplied by the HMRC Advisory Fuel Rates, which are determined on your engine size and fuel type of your vehicle. The calculation will look something like the below.
If you're in a 1498cc Petrol Engine vehicle, as of April 2025 your AFR is 15 pence per mile
Personal miles for the April period = 250
0.15 x 250 = £37.50 deduction due.