Hi Shahinoor, I'm in a similar position to you, we are in Erith.
DG give priority to those living in Dartford parishes, other than that they cream off the top scorers.
From last year's results which have been posted, and other posts regarding DG, they went down to offering places for scores of 411 (which is pretty high!), so I guess we should be aiming at getting 140 for all 3 papers to give a top score of 420 or not much less.
Depending on age, that means a percentage of around 70 -80, slightly less for younger children.
And seeing as we can't really equate practise tests to the real thing I guess we aim as high as possible!
Good luck to your DS, will he be taking the tests at Wilmington?