The process is designed to allow to enter their vehicle odometer readings at the point of submission. The software then calculates the business mileage total for the header and infers the difference between the last odometer reading as personal mileage. The cumulative approach means that values cannot be amended when unsubmitted.
Some points to consider are below:
The mileage rate calculation is completely independent to how the system calculates your HMRC mileage rate for your vehicle class based on miles travelled in financial year. This totals the mileage for claims posted rather than using the values in the closing balance field.
If claims are resubmitted using the original header, then the mileage is not added a second time.
It is possible to amend the closing balance when submitting the claim.
It is also possible to submit a claim with a negative mileage quantity which can be unsubmitted and deleted to redress any imbalance.