I don't know which particular schools you are considering, but I can only comment based on my experience of boys' grammars in Sutton.
One of mine missed SGS by a couple of points. His ranking from Sutton mock was at about 30% from the top and his score in the real exam was a few points higher than in the mock. According to his results letter I received, 28% of boys that year achieved the required pass mark, so with just the two marks below that, DS's ranking in the real exam was pretty consistent with the mock.
What you need to bear in mind is that the number of boys who achieve the pass mark and are therefore eligible for a place in one of Sutton grammars, is always greater than the number of places available. When DS1 did it, 28% of the cohort equalled to 417 boys and SGS had only 120 places available at the time, and there were, I think, 330 places available for all three boys' grammars in Sutton (I remember thinking 90 boys would have gone straight to waiting lists).
After the last round of exams (children currently in Y6), ca. 550 boys passed SGS and Wilson's (Wallington passed 781 after SET) and they were qualified as eligible for the total of 450 places. This is roughly half of the boys who passed SET (Stage 1). I'm not sure how many sat the SET in September, but I've heard of over 2500 boys, so if there are 450 places available for boys in Sutton, a boy would need to be in the top 18% to be offered a place.
As far as mock ranking is concerned, for us it proved to be consistent wiht the real exam scores. If this is the case, you would need to aim for a minimum of top 15% from the mock to have a glimmer of hope and no lower than top 10% to feel a bit more comfortable about your DC's chances.
In case it may be of use, information below comes from emails sent out after the last round of exams: Sutton Grammar:
913 candidates sat the second stage test and 551 have met the required standard. For candidates who have met the required standard, no information regarding their marks will be given to parents. Please note that, due to over-subscription, meeting the required standard does not guarantee the offer of a place at the school. Wilsons's:
851 boys sat, and 543 boys have passed, the entrance examinations for Wilson’s School.
Good luck, I hope it all goes well for you.