You have hit on the $55,000 question!
Firstly, the good news, in the past children who have got a place at Pates have mostly been scoring consistently in the 90's in practice tests. That said, and here comes the crunch...it depends how well all the other kids do in the exam who wish to be considered for a place at Pates.
No-one alive can say that just because your DC is getting 90's that they will on the day achieve the same (all manner of factors!) BUT I would be more connfident of a Pates place if my DC was getting those scores on the doors! Also, I am not sure how many tests you have done or if they were timed etc. Getting 90+ in four untimed tests is not going to give you as true a picture as getting 90+ in some of the hardest tests and strictly timed, if you get my meaning.
More good news, if your child is younger they do get consideration/compensation for that fact. How it is worked out is some kind of mathmatical nightmare which perhaps someone else can explain better than me should you wish to know.
This has been discussed on the forum before so if you trawl through you will find some more info.viewtopic.php?f=16&t=11364
this link might scare or comfort you depending on your personality traits :0)
Hope this helps a little
Good Luck and Best Wishes