InfoPath Dev

Hi. I have a monthly form for our service guys to enter their fuel purchases and mileage in a repeating table. The problem is the mileage.In theory to calculate mileage you subtract the last entry's mileage from the previous months closing mileage (the current month's opening mileage). I have a text box outside the repeating table for the opening mileage. How do I subtract the last entry in the repeating table from the opening mileage?

There might be an easier way but you could add column to your repeating table with a text box in it. Put your formula in the "value". That way each time you add a row you'll have a new calculation showing the mileage.