Thanks for the replies, which have prompted some more questions 🙂

1) I've already run the first 80GB tlog backup and was under the impression that every subsequent tlog backup would be the same size until I used a SHRINK on the tlog? I can't efficiently test this because I would run out of space if indeed the second tlog backup was the same size.

2) My idea before was that after a SHRINK I could control the size of the tlog with regular tlog backups and just leave the growth increment to the default (10%). You said that I shouldn't let it grow 'dynamically' by this do you mean that I shouldn't SHRINK it to a really small size and let it grow by itself? This would create a performance issue?

3) The advised procedure on SHRINKING the tlog after the initial 80GB tlog backup includes a CHECKPOINT. I understand this writes the 'dirty' pages from memory onto the disk. What's the benefit of doing this before the SHRINK? Are there any negative impacts of running the CHECKPOINT (I'm guessing it doesn't break the backup log chain)?