My V/D calculator comes up with the same thing so I think you will be OK on #10 and I agree on the 15a breaker. I have a 1/2hp well pump that is doing fine on a 15.
Them pushing this out to 1020' may be a stretch but it still only works out to about 5% V/D (12v from 240). I am guessing that is where the number came from.
