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.
No, we don't have a way to do that. I know it's frustrating that you'll have one fuel-up without a calculated MPG, but it won't make much difference in the long haul. If you need more control than odometer readings provide, you can always choose tripometer readings instead. That way every fuel-up will include a calculated MPG.