Return the Percentage of Characters in String A that occur in string B

I'm looking to create a function that returns the percentage of characters in string [A] that occur in string |B|.  

Example

String |A| = 'bookmarked'
String |B| = 'bookstands'

Using string |A| as the base, 50% of the characters in string |A| occur in string |B| (b,o,o,k,a).  The percentage should be returned from the function as an integer.  

Thanks, Gurus.