At the moment, we work our mileage out "as the crow flies". Imagine a straight line on a map from you and the customer. So it is likely the actual mileage will be a little further depending on how bendy the roads are in your area. Luckily, this is something we took into account early on and so we increased our extra mileage pay to 90p extra per mile to compensate for this.
We are looking into getting a more accurate millage tracker that takes more variables into account in the near future.