transfer rate calculations

The backup window is the time when you can do your backups.  Backups may affect other tasks/jobs and vice-versa.  Hence you would only do your backups during a quiet time when there are no other tasks/jobs running.  This is the backup Window.  For example, after your office closes at 5 p.m., you may be doing batch processing and/or housekeeping jobs until 9 p.m.  Hence you cannot start your backups until after 9 p.m. Your office opens at 9 a.m. in the morning and you don't want your backups to slow everybody down.  Hence your backups must complete by 9 a.m.  The period between 9 p.m. and 9 a.m. is your backup window.

Don't worry about the calculations.  These are theoretical calculations used to choose what backup technology you should use, so that you can buy the right equipment.  Obviously, if you have a 12 hour backup window, you cannot choose a technology which cannot backup the amount of data that you have within this 12 hours.

Since you managed to complete your backups within your backup window with your LTO4 tape, you are fine.