Welcome, MPG Junkie. I too am obsessed with measuring and recording my mpg. I assume you calculate your mpg as the onboard display, known on this site as the "Fibometer", is notoriously optimistic.
As well as calculating and recording my mpg in a spreadsheet, I also use two online sites.
Spritmonitor.de (German site but with an English section as well), whose figure you can see beside my avatar. This figure is my average during my ownership of the car (currently approaching four and a half years).
The second site I use is Fuelly.com, and some of the other contributors here display their numbers beside their avatar.
Every time I top up, I switch off at the first click of the pump and input the litres used and the odometer reading, as well as calculating in my spreadsheet. (I know this is a bit CDO - OCD but in the correct order - but as you and I admit, we are junkies).
I also have a ScanGauge E fitted to my OBD II port, and this displays the instantaneous mpg (great for maximising DFCO) and the journey average. It can also do a hundred other things, as well as being a Code Reader and resetting tool.
The benefit of the ScanGauge over the "Fibometer" is the ScanGauge has a correction applied every time you top up and so returns a more accurate (though not perfect) mpg figure.