If you use the same technique each time the fuel economy will show that one more gallon of fuel was "used" to refill the first time with a false lower MPG, accurate numbers will be produced from then on, (or until you cease the adding of the extra gallon). That fill up without the extra gallon would then indicate that one less gallon was "used" to refill after the miles driven on the last tank and falsely indicate a high MPG.
In the long run it will all even out.
Oh, I forgot about that part of the question. I sometimes have to put in a partial fill. I save the receipt and don't reset the odometer; then I add that quantity of gas to the total amount for my next fill. If you're putting in exactly one extra gallon, just ignore it until you do fill up, then add one gallon to that total.
Partial fills make your MPG numbers more accurate. A pump that will click at the same point in the tank +- half a gallon can be off by 5% in a 10 gallon tank but if you partial fill the tank 5 times with 8 gallons and then fill it from empty you've reduced your margin to 1%.
In your case the extra gallon works the same way unless the aforementioned emissions stuff sucks up the fuel. That should show in your MPG numbers.
Also depends on the car and how the fuel fill necks shaped. I had an old 86 camry that you had to fill on the lowest notch of the nozzle then put the last 2 gallons in manually. My truck can take under .25g after clicking off. My cressida takes about 1-1.25g after clicking off.
As far as calculations, as long as it's measured it counts. so no worries with the math.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"
I was getting close to empty, but my wife thought we'd get a really good discount on some gas at this one place. I put in 1 gallon so we could get there.
We got there and it was a pain in the butt, so after having prepaid $30, I decided I didn't want to waste more time and I'd rather just get home. So, now I had an almost-full tank.
Finally, the next day I went to yet another gas station and topped off so I could have a complete fill. I added it all together while my wife sat next to me impatiently tapping her fingers/feet/etc and complaining...