I don't think you will ever find a stated tolerance. The key to getting a warranty claim honoured seems to be choosing the right dealer. Some appear to be better than others at getting Vauxhall to accept liability.
I can't say, and as Robin says it's probably as much to do with the dealer as anything.
For instance, Ford refused to even contribute towards a trashed engine of a 2 year old Mondeo because I'd missed a 30k service by about 2k (all figures approximate!, But i think it was less than 2k). There was some evidence on the internet that the problem was not unique to me (it was the 5 cylinder 2.5 Volvo engine, also in other Fords)
Yet Kia paid for diagnosis and replacement keys (incl programming) at about 7y 6m (i.e. 6m after the warranty) when my daughter's high mileage (and not Kia serviced) Kia had an issue. (The keys had hairline cracks near the fob which meant the immobiliser couldn't be deactivated).
And Vx refused to contribute to the cost of a pair of cracked (original) tyres on my 4yo Amp last year, until I involved the ombudsman. They then grudgingly gave me "50% off" IF fitted at their dealer, which mean the tyres cost ne about £10 or so MORE than they would have done at the local tyre emporium.