Yeah, without an odometer reading there's no way to know how those gallons were used. If you still want to remember the details of your fuel-up you can add them as note. Just be sure that the next time you add a fuel-up, check the box next to "I missed a fuel-up" and you'll be set.
Another option is to estimate how much you drove after you fueled-up and subtract that amount from your current odometer reading. It won't be perfectly accurate, but sometimes if you haven't done much driving it can be easy to calculate.
That's what I did. Estimated & checked the partial fuel up so it wouldn't calculate an average.
Can't you just let it skip? Force it to be marked partial if there is no odometer reading. You will have the odometer reading on the next fuel up and the total gallons from 2 fuel ups would still be there.
Why check partial? If it was a full tank there's no reason to call it a partial tank.
No, we want every fuel-up to have a distance and fuel volume. Changing that complicates the system too much. It wouldn't be too far out of the realm of possibility in this one instance to skip an odometer reading and do some sort of combining on the next fuel-up. But keep in mind we are storing thousands of fuel-ups with hundreds of different scenarios. By keeping the simple rule that every fuel-up needs to have a distance and fuel volume, we're able to handle all of those scenarios. Sorry it's a hassle for you in this case.