How on earth do you define "average"? Every setup is different, and how tight it needs to be depends on how the part is being held, the part shape and material, what machining operations you're performing, and how aggressively your' doing it, among other things. Those factors alone can easily account for a range of 10:1.
My experience is "witness marks" are much more a function of how you hold the parts, rather than how hard. Screws, properly used, won't leave marks (i.e. - use washers, or put the screws where the marks can't be seen on the finished part). A vise, when tightened, will almost always leave some kind of mark, unless you either garb the partly lightly (which is NEVER a good idea!), or use aluminum soft jaws. IMHO, well-designed, dedicated fixtures are the best way to make mark-free parts.
Regards,
Ray L.