Unfortunately none of the conversion sites will give accurate values, there are too many variables.
You can find some more topics on
Value of old money in the
RootsChat Reference 
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Lexicon (click here)(Tip: click on a category - on the right - for related topics)
These topics discuss some of the many variables that Guy has mentioned, and also various comparative values.
e.g. the price of a pint of beer, wages (through the ages), property values, and many others.
Bob
