we were told by a local tutor with a lot of experience that 75% was about the required level. 334/420 (ie an Alty pass)comes out at just short of 80% but looking at my dds scores in the exam they were all a bit higher as percentages than she was acheiving pre exam, so I think the 75% is probably about right, the beauty of course being that if they have one strong area they can slip a little in the others (unlike some 11+ areas where there is a pass mark in each subject)
My dd was 10 yrs 8m when she sat the paper but I don't think the age correction accounts for much, will just help if the result is very borderline.
The Sale exam was passed by dd with a much greater margin in all 3 disciplines, 3 weeks later, with no work in between. Wouldn't like to hazard a guess why this was!
Hope this helps, and good luck with the tests