Add a "Beginning Odometer Reading"

[Buxwheat]

Can I add the odometer reading before my first fill up (using my trip odometer to calculate the starting reading) to start generating data with my first fill up? In other words, until you fill up a second time, you can't see your current MPG, but if I know my starting point and fill in today's odometer reading and gas purchase, I would start out with some data.