Sorry not familar with these levels but if the are based on SATs they are not that relevent, especially in an International School where they have lots of children with English as a Foriegn Language (and actually DD1s bright kids on the top table included a Finn who had only arrived three years before with no English but still outclassed most of the reading group I ran). They have uselessly broad boundaries and are all about achievement rather than ability, intellectual curiousity etc. the indy prep schools pressure these kids to get them to the top of the tables but they don't serve any other purpose. The good schools go out of their way to spot ability rather than achievement.
What might help you find DS's level, if not comparative with his class, is doing some verbal and non verbal reasoning, they are tests of ability rather than achievement and good schools include them in the exams. I found NFER best but that was a while ago, the key thing is that the score gives you a percentile of the overall population of his age group in the UK that your son's score has exceeded. Sorry Girls' schools again, but Lady Eleanor Holles, generally regarded as next most selective for girls after St Pauls in West London were saying that they expected that girls who got in would score at higher than the 95th percentile (also the figure for Russell Group Unis and Oxbridge), for Tiffin (state grammar school )it was the 97th. You should do a few practise tests first because within a quite low ceiling they do better with experience (I used to work with Occupational Psychologists, it was Scientifically proved this ceiling was 6-10 tests not that you would know it from all the tutors minting it on repeated tutoring and tests!!)
However as I said in my last post these schools are after more than just cramming, they all have a particular ethos and they know who will thrive there so they test and interview to find a type. I know it is difficult coming from overseas because your school will not say he's a St Paul's boy or you should go for..... but actually coming from abroad also gives him a big advantage, especially at the schools that don't have boarding. I know this sounds trite but you will know when you walk into a school if you are not too obsessed with league tables, results and the status your child being wherever will give you at dinner parties, whether it is right for your DS. I found it hard to trust but DD1 knew and she was right. DD2 had to prove she could succeed where DD1 had, but I felt another less selective school felt right. Guess where DD2 is going for Sixth form. Trust your instinct.
Schools also publish sample papers so that gives you some evidence on which to base a judgement .
Kingston Grammar School is great, don't hear any complaints, happy students and v. impressive Oxbridge, medics entry. Word is it knows how to stimulate and get the best potential from it's intake