Find the value OPTIMIZE FOR UNKNOWN is using
OPTIMIZE FOR UNKNOWN doesn't use a value - instead, it uses the density vector. 

If you run DBCC SHOWSTATISTICS, it's the value listed in the "All density" column of the second result set:



In this example, I'm using the StackOverflow demo database. The density vector for the Reputation column is 5.962674E-05.

If you take that value, times the number of rows in the table, 5.962674E-05 * 5277831, you get 314.69985680094. That's the number of rows SQL Server will expect to come back for any given Reputation filter.