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Try to tighten up the emphasis on the consumer a bit, perhaps -> just make sure the differential is there to avoid any potential duplicate detection issues.
Outside the engines, you can make an educated guess, but to state anything such as 'x % similarity will result in banning' is difficult, as few if any people outside search engine employees will have that data.
Any situation is unique unto itself, and not knowing all the variables I am very uncomfortable giving a blunt & specific answer.
of course, I would love it if the search engines told us what % similarity was ok, but that would more than likely lead to more people trying to 'manipulate the search engine' and 'artificially increase their rankings'. :)
I know, I'm being vague, but the most you can do is make an educated guess, based on all the statistics, etc - and then go with your 'gut' as delaying an important business decision - rightly or wrongly, could cost $$$.
Or when in doubt, hire a professional. That way, worst case scenario, they take the blame instead of a 'staffer'.