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I don't think this is a dangerous topic, but maybe this is a dangerous answer.
Yes, This is a better response. I stand corrected!
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For reasons already mentioned, I don't think it is possible to say that there is a blanket percentage of Scott value that can be applied to any group of or individual stamp. There are just too many variables (supply, demand/popularity, condition, hinged or not, etc.).
Much better explanation.
To give an example US Scott R2c(this is a scarce stamp but far from rare). 85% are poorly centered and sell from 85-100% of Scott. A VF copy sells for 100%-120%. A VF with a Playing Card Co. Hand cancel sells for 125%++ depending on Mfg. Any XF examples at auction will go 200% or more(w/o premium for cancel). Cancel also plays a large part in the equation.
Classic Germany up to the end of WW2 is another area that you can throw Scott out the window.
Bottom line I would say if it is more than a few dollars(or what ever limit you set) do your homework and shop around. Personally I keep a spread sheet with my want list, it contains Cat. Value, My fair market buy price range by condition(again do your homework). Below that range look for hidden thins, re-perf., etc. Beyond that buy range I let it walk(I'm in no rush to overpay).