If you browse the big list of certified rides at
https://www.ironbutt.org/certified.htm you'll see plenty of certified rides 2 or 3 miles over...
The verification team will calculate the shortest possible distance (within reason, of course. Goat trails not likely used) between each of your receipts using tools such as Google Maps. Regardless of what actual path you took between those points and the value displayed by your odometer, that will be your "official" mileage. As such, it would behoove you to do the same... As long as that value exceeds the mileage required by the certificate, and you make sure there aren't any short cuts unaccounted for between your receipts, you're golden.
That all being said, I am typically comfortable with 20-30 miles excess, providing I'm not pushing the DNF time too close (in case there is a problem obtaining a valid receipt at the end).