got it. actually, you can correlate the LSNs in the backup files directly with what's in the msdb..restorehistory. The query at the bottom of this article:

http://www.sqlbackuprestore.com/logsequencenumbers.htm

shows what you need:

SELECT TOP 1 b.type, b.first_lsn, b.last_lsn, b.checkpoint_lsn, b.database_backup_lsn

FROM msdb..restorehistory a

INNER JOIN msdb..backupset b ON a.backup_set_id = b.backup_set_id

WHERE a.destination_database_name = 'AdventureWorks'

ORDER BY restore_date DESC