I have a son in Y10 and my understanding is that everyone who passes the Kent test and lives in their designated catchment area A will get a place if they apply (and they're a boy, of course)
You can double-check the area covered on their website
After that, as Bondgirl says, they work on score going down (presumably from 423), and where there is a bunch of applicants on the same score, on distance. So there is going to be a point at which some boys living nearby on the lowest score for that year get in, and boys living further away with the same score, start filling up the waiting list.
I don't see anything in DGS' admission criteria operating a top 25% guaranteed place, like hermanmunster I think that figure must arise from the estimate that children in the top 25% ability band nationally might be expected to pass the Kent test.
It is an exceptional school, and I'd say that in ds' year, about 20% are not from Dartford. Ds was a successful OOC candidate and he has classmates who commute in from Mottingham, Chislehurst, Greenwich and even Beckton
. But most of them are from Dartford, Greenhithe, Crayford etc.