SQL Server log file used pecentage does not drop after multiple backups
In addition to the additional space marked "in-use" by SQLPERF you might have space in your transaction log that is not being freed for other reasons.  You should probably take a look at the log_reuse_wait_desc column of sys.databases. If it says NOTHING or LOG_BACKUP then what you are seeing is probably the space taken up by the current VLF.  If however if you are seeing other values, such as DATABASE_MIRRORING or REPLICATION then a portion of your log is waiting to be replicated to another machine.  VLFs are not marked "free" unless they are completely ready to be re-used.  This can mean an open transaction or waiting on a log backup (if in bulk-logged or full recovery model), but it can also be other things too.