Roses, I am not sure how helpful my views will be given that we are also going through the process so no past wisdom, but for what it's worth - I think if you are getting 85%, you are a slam dunk. And 70-80% is a pretty good chance too.
Where does this conclusion come from? I have actually done 5 maths papers under timed conditions - I have finished on time, but only just! I have done a highly quantitative subject at university (although not maths) so I am very comfortable with maths, but for some questions, I had to pause and think. And obviously as an adult you don't make silly mistakes, work much faster, often see the method to use immediately, etc. Based on the above, I would be extremely surprised if there were more than a handful of 10-11 year old girls (if any!) who finish the maths paper AND do all the tasks right. English is obviously less measurable, but again if she finishes practice papers on time and answers have reasonable depths, you should be fine. English is more of an anxiety for us, because DD is bilingual, borderline dyslexic, and not a voracious reader. Plus very analytical so writing about "how the character feels" in great depth is a bit of an issue, but for any "normal" bright girl, English paper should not present a particular challenge.
Also, as shootingatpigeons said, girls sit for 4-5 schools but will only take one place. So a girl who is offered G&L, would often have LEH, SPGS, and Latymer, but can only take one place. At least 20 girls from Bute House alone will turn down G&L for St Paul's. I don't know if G&L overoffers by a lot or all go onto waiting list, but my point is that if your daughter is a reasonable standard in exams and it's genuinely your first preference and you let them know, you should have a decent chance.