Proper resizing of Log File after shrinking - SQL
I think you'll find a good starting point here - Set transaction log size and growth

To summarize from the link:


Set your transaction log large enough so it will not grow during
normal operation. All update activity stops in the database when the
logs grows, so you want to minimize log growths.
For an initial log size, you could roughly estimate the log size
needed to rebuild your largest table in db (roughly, that is the size
of that table) and set the initial log size to that value. That would
be the “fit largest transaction” method. Imprecise, but is very
simple.
Log file growth should be changed from a percentage (as is the
default) to a fixed MB size and monitored.
The link references the usage of BULK RECOVERY model to limit the
size of the transaction log due to 'minimally logged' transactions
like BULK INSERT and INDEX REBUILDS. You can only switch to BULK
RECOVERY from FULL RECOVERY, but you should be aware that you cannot
restore to a point of time that encompasses a 'minimally logged'
transaction because individual log (insert/update/delete) records are
not generated during that process. While the actual log file will not
be large for a 'minimally logged' transaction, the log backup will
still be large due to backing up the changed data extents that were
affected during the 'minimally logged' transaction.
In FULL RECOVERY, the transaction log will continue to grow until a
log backup occurs (assuming there aren't any open transactions left)
In SIMPLE RECOVERY, the transaction log is 'basically' available for
reuse after a transaction commits work.
Generally speaking, shrinking the log is discouraged. At some point,
there was activity that caused it to grow to that size (index
rebuild, etc.). There may be situations where an out-of-the-normal
issue occurred and caused the transaction to grow large than normally
needed and in those cases you may need to shrink the log down to a
more realistic size




On a side note, your question states you are taking transaction log backups 8 times per day (6 days per week).  I would highly encourage you to read Back Up Transaction Logs Every Minute. Yes, Really.