Need your opinion on calculation error
 

Ok so I got my very first fuel log for my 97' accord and it was 15 mpg, and I was like WTF? and figured it was because I had been blaring the A/C all week...

Hoping that the trip was off I looked at the first record I did of the car, it said I did the "maintenance" at 153,274 miles... I subtracted this from the miles of my fill and it said MORE than the trip said the whole tank was...

So the trip must be off and I need to start reading off the odo.. What is the best way you guys think I can calculate the miles that were on my first fill up, I was thinking at my next fill I take the miles down (say its 200m) and take the trip (say its 150m) and calculate the difference (which is 25%) and so id just add 25% of the trip number to itself (hoping it stays consistently wrong) haha

fill the car and record the odo.

during your tank drive a known distance and compare to the distance your odo says. do math magic to get a % of how much your odo is off

Next fill record gallons and odo subtract old odo from new odo to get miles. multiply miles times % then devide by gallons