Chinese Stamps On Cover

For a given Chinese stamp, Scott gives in its catalogue an evaluation for a used one ( cancelled).
However, this is for an isolated stamp.
When the same stamp is used on a cover which has circulated, it must have a bigger value.
Neither Scott nor Michel gives such evaluation.
How can I estimate the additional value due to the cancelling on cover?
Is there a rule?
Thank you

*** Moved by Staff to a more appropriate forum. ***

hi; The best thing is look in the listings of auction houses(interasia & others,e-bay,Delcampe)to get a good idea.Depends on the Chinese stamp;PRC or imperial,postmarks ect...
They come in thousands of forms & catalog Quotations would be impossible IMO.

For modern era stamps such as most the of the PRC ones there is little premium for a stamp being on a cover, but rather the value is in the cover itself as postal history and how and where the cover moved through the postal system. For older stamps such as the pre-war and especially the 19th century and early 20th century this postal history value can be quite significant but there is a range depending on the nature of the history of that cover - did it go by an interesting method of transportation, are there extra markings beyond the basic cancellation, was it sent by a famous person, etc. A stamp that is on a relatively plain and uninteresting cover will have less premium than one on an interesting cover. And often the value of the stamps become secondary at best to the value of a cover that is real postal history.