Tor periodically logs a diagnostic message with the percentage of TLS write overhead. How can one determine if the percentage reported is indicative of a problem with the associated tor relay?
So far it's just something that might be useful for developers to collect. It's probably nothing that an individual user or relay operator would have to care about at this time.
It's also not even clear yet what the answer to your question really is.
As Roger explained in What is the "TLS write overhead" percentage reported in Tor log entries?, Ticket#7707 is where some of the development discussion happens. If you are interested in it, more details can be found there.
In short, nothing to see here, move along. :)