You intended to use Actuals, but you still need to know how much of the Actuals was business, if there is any mixed use.

"I can get an average for the miles"

It's not "average." It's supposed to be documented, not estimated. If the vehicle was used solely for business, that makes it easy. Check any maintenance and repair receipt, tire purchase, rotation, etc. They all document the mileage on that date of service.

If they have a routine travel scenario, perhaps they have a schedule from work assignments that will help them create a mileage log from a work log.