Guideline for MySQL free space
A really poorly constructed query can take a huge amount of space -- can't really protect from that.

Your maintenance?  Or AWS's maintenance?

If you can 'quickly' upgrade to a bigger instance, would that be an acceptable fix for nearly running out of space?

Would you like a review of the schema for the largest table?  Maybe some datatypes could be shrunk.  Maybe some normalization is feasible.  Maybe unhexing uuids, etc, is advisable.  Etc.

Otherwise, stick with your old rule of thumb.