You can use this as a basis ...
http://t206resource.com/Article-T206...ltipliers.html
However, as stated in the previous post, some more common backs can be extremely difficult to find on certain card fronts. Here is a post Ted did regarding Old Mill backs on the "Exclusive 12" of the 460-Series only cards ...
http://www.net54baseball.com/showthread.php?t=194236
Hope this helps some,
James