This is probably not what you want to hear, but there is no exact way you can convert your score on a practice paper at home to the standardised score your child will achieve in the real exam.
Remember that there are only a certain number of Grammar School places available each year and the number of children competing for those places can also vary each year. The important thing is to determine how well your child did compared to all the other children. If your child is in the top 15% (not an exact figure but a reasonable guideline from what we hear) compared to everybody else then they are likely to pass. This is why we provide the percentile rank on the feedback from our
11+ mock test days. If your child's percentile rank is above 85 then they scored better than 85% of people taking that same test and are
likely to pass.
Because pretty much ALL entrants are getting some form of coaching for the 11+ nowadays, the level of competition is higher (that's a good thing btw - makes the test fair). I think it will eventually mean that the test has to become harder, but that's another story.
This sounds like a stupid thing to say, but unless your child is scoring 100% in every practice paper, there is still room for improvement. Keep practising steadily but don't overdo it. Be supportive and caring and let your child know that you love them and will always love them whatever the outcome of the test. Just work on keeping them calm but, at the same time, excited to do their best.