Previous posters are absolutely right about it all being about how well the cohort of students who sit that exact test, in the same conditions, on the same day perform i.e. there are no "rules" regarding pass marks just are there are not in public exams, it's all relative!
BUT, I remember wondering this myself before DD1 sat the 11+ last year and hope that my experiences with her might help a bit....
She did not have any formal tutoring, but we spent some time over the months following registering for the test going through some Maths with her (as we knew from friends that the test covers things that are not taught at school until Year 6 and beyond) and doing some of the "10 minute" tests in the CGP mixed workout book (only one we bought as it covered the different elements of the test) and she did about 4 of the CGP practice papers which we gave her to do to familiarise herself with the format of the papers, filling in lozenges instead of writing out answers, timing of sections etc.
She was scoring between 65-80% on the CGP 10 minute tests (lowest score was 45% and highest 89%) and averaged about 75% on the practice papers. She qualified in the top 150 for all the girls grammars and Pates and is off to the latter in a few weeks time.
I would, however, absolutely not get wound up about pass marks. As I've already said, it's all relative. However, I know what it's like not having any benchmark. I am a teacher myself and know that I am constantly on the hunt for benchmarks for my GCSE / A level students and used to work out average marks needed for each grade on each paper from past papers before the reform to GCSE (have started again now!)
The most impactful thing we did with DD1 was to look at exam technique with her. I know that this - above anything - enabled her to perform to the best of her ability on the day and gave her the confidence to take it all in her stride. DD2 and 3 will be sitting the test next September (2020) and will do exactly the same with them. Nothing at all til June then focus on exam technique above all else in the couple of months before the test.
Hope this helps. I appreciate it's just my opinion / experience but - as I've already said - I took comfort from other's experiences when I was going through the prep period last summer
All the best for September