How to find the current Log Sequence Number (LSN) of a database that is in Restoring state?
If you restore the log backup with "HeaderOnly", you will have the first and last LSN include in this backup.  

You want to find the log backup that includes the LSN mentioned in your error message (where the DB is at). 

Now if you want to find this LSN, you will have to query the sys.master_files DMV and look at the redo_start_lsn column.

Select redo_start_lsn from sys.master_files where database_id=DB_ID('yourDB') and type_desc = 'LOG'