When should you shrink your transaction log?
Transaction log sizing is important also because of VLF fragmentation which could wreck havoc on your restores and transaction log performance.  Ensure your tlog VLF isnt heavily fragmented first I'd say.
Secondly a good way Paul Randal stated was to let your app run natively for a week, and see how big it gets. Make sure you do your regular maintenance on it and such.
Now maybe it gets so big because of ad hoc maintenance, queries, badly written queries, but to find the current good size that would work.
https://www.sqlshack.com/what-is-sql-server-virtual-log-file-and-how-to-monitor-it/