Usage based billing based on month and year both

We have specific business scenario, where we have agreement with customer to bill on minimum of 300,000 KM per year per bus, this billing should be done on monthly basis let say 25000 Km per month usage should be billed, but at the year end we have to calculate if the actual KM during the year exceed 300,000 if so, we will bill the additional KM otherwise if actual KM during the year is less than 300,000, we do nothing because the minimum quantity of 300,000 is already billed monthly

for this scenario we are using USAGE based billing in subscription management, but the we are unable to figure it our how we can map this requirement on subscription