Chained Rows Read Ratio ( formula )

What "Chained Rows " means is clear so far. Here is a good description: arkadio.com

But is there a formula for the evaluation "Chained Rows Read Ratio" ?

In the AWR report a lot of statistics exists ( Source: Statistics ) :

• tfcr = table fetch continued row (Chained Rows)        : Number of times a chained or migrated row is encountered during a fetch
• tfbr = table fetch by rowid     (Index-Use)           : Number of rows that are fetched using a ROWID (usually recovered from an index)
• tsrg = table scan rows gotten   (Full Table Scan Rows) : Number of rows that are processed during scanning operations
• slrd = session logical reads    (sum "db block gets" + "consistent gets" ) : The sum of "db block gets" plus "consistent gets".