Hey GW. It's probably not the most accurate, but the way that I do it is reset the trip od. every time I fill up. I make sure the machine spits out a receipt when I am done and I write down the trip reading on the receipt. Then, do exactly as Blue Bowtie said. I take it a step further and keep an Excel spreadsheet with all the data.
It may not be the most accurate the first few times, but if you continually collect the data over a period of time, statistically, the standard deviation will decrease the more samples you take.
Microsoft has
these templates. I use one that is slightly modified. It's on my computer at home, so I'll edit and attach a copy later today.