MPG calculation

[RobertV]

Originally Posted by teamhonda

Because I could with the previous version - often times one's time is limited - I'm adding a few gallons and get on my way due to time constraints or very inclement weather.

What you're describing is a Partial fuel up- where you pump a few gallons of fuel, or any amount other than completely filling the tank. In which case, put your current odometer value in for that fuel up and mark it as a Partial Fill-up (there's a checkbox).
You can do that as many times as you'd like, as long as you continue marking Partial, it'll wait to calculate your MPG until you've logged a full fillup (completely fill the tank). At that point it'll combine all the mileage you've traveled from all your previous partials, and all the gallons you've pumped and calculate your MPG.

Originally Posted by teamhonda

I enter the info in the app for the family and at times my spouse forgets to write down the odometer reading and the overall mpg can be tracked with the next fill up when the odometer data gets entered.

This sounds more like a "Missed" Fuel up. If you don't have all the information for a fuel up, just wait until your next full fill up, and mark it as "Missed". This will reset the calculations to start from that point.