Thanks Spooky - I think the closest we can come in European Ops IS (?always was?) contingency which is (?was?) 5% of flight time for turbine and 10% for piston. This has now been 'alleviated' (if approved) at airline request to a 'statistical' figure with a bottom stop as Denti says of 3% or 5 minutes whichever is greater.
When I finished JAR/EUOPS flying, including ETOPS, in 2008 we had no requirement for a 10% figure, but other (separate) factors were considered. I know not of now, so treat the above with caution.