Fact : I have several SS1K's that have been certified with less than 1,010 miles. But remember, it's your responsibility to prove you did at least 1K miles. Don't rely on your odometer, for sure.
BTW, I use Streets & Trips to plan my rides and S&T and my Garmin 2720 after the ride have always agreed within 1 mile.
I know Ira won't like this ............. 2% of 1,000 is 20 , but it looks like you don't need that much of a cushion if you have an accurate routing program to prove you will do over 1K miles. And as far as I know, the IBA has never said what it uses to verify the mileage of a route. I asked once about that, but didn't get an informative response. But I think I can rely on Streets & Trips to give me an accurate mileage. I feel about 5 miles is a plenty good cushion when using an accurate routing program. So far, no problem getting my rides certified.
Incidentally , after one of these SS1K rides, my Garmin 2720 trip meter read 1,008 miles, and my riding buddie's trip meter on his Can Am Spyder read 1,008 , too. He was Ward Blanchard ( RIP ) and on that ride he set the record for the oldest Rider to complete a SS1K at 89 years old. Ward passed this January.