Can I enter in only unreimbursed mileage instead of total miles?

My work only reimburses us for part of the standard mileage rate.  Is there a way for me to only enter in the difference instead of having to enter in beginning mileage, ending mileage, total miles, etc.  None of that is needed and would not be accurate.  For example, they reimbursed us for 60%, I just want to claim the remaining 40%.