So instead of puzzling about how scores on practice papers will equate to real test scores, to which you will never find a satisfactory answer concentrate on the following:
1. Is my child roughly in the top 25% of the population and is grammar school the right place for him / her?
Totally agree with you on this. in that if the child is not working at the top level in the class they will probably struggle in Grammar.
However, the problem in kent is that as we have to apply before 11+ is sat and we can only choose 3 schools it can be quite worrying. Despite my daughter being the top girl in her class I worry endlessly about the Maths exam. The only non-selective we are close to and assured a place in is appaling, a very rough place in whcih I'm sure my bookish daughter would be fried alive! I therefore panic about her failing and having to appeal to the better comp on my Caf form that is very over-subscibed and further away.
Sometimes seems like a lottery to me. What a horrible system this is