I would go for 25%, but I would change the exam and move it to 13.
I did CE at 13 and won a place at a grammar school in Kent via that. CE has papers in all subjects - the VR I actually didn't realise we had sat as it just got shoved in as a 'quiz' one morning.
It meant that if you were crap at maths but outstanding at history, geography, English, French etc it could be taken into account. Likewise the mathematicians could let the English slide a bit. I still had to get 75% average across the board to win a place.
Obviously this would require a lot more work in terms of marking, and it wouldn't prevent people tutoring (I will hold my hands up and say that my parents paid for a hot-housing prep-school for 4 years because they couldn't afford an indie at secondary level and the local comps were truly dire).
It would also require an extra 2 years in primary schools.
The non-grammars should then cater for the full-range: top sets for those children who either didn't wish to sit for the grammar or those who didn't get a place but are still academic - could give many children who wouldn't otherwise have the chance the possibility of shining... which can only be good for confidence and inspiring greater effort and attainment.
There should be an option for children who score particularly well at GCSE to move at 16 to the grammar for A-levels should they so wish. And for those who aren't really thriving at the grammar to move the other way.
Perhaps there could be ways that certain subjects could be taught together - a bit like the all-girls indies sometimes pair up with the all-boys indies for some things.
For the less academic there should be intensive teaching on things like English and Maths, with the curriculum directed at the workplace... sod quadratic equations, lets just have the kind of skills you might need in an office or as a builder for example.
For those who are really unenamoured by school there should be lots of practical training.
Above all, lots of money into everything - and especially at the lower end.
The whole failure/success things is a British issue that just doesn't seem to happen in other countries to the same extent. No idea how to solve that, just seems a big shame and very sad.
Anyway... that is my idea for a good system that includes grammar schools.