Check out some books and online articles on the topic by Joe Friel and Andy Coggin. Chad referenced books by both of them recently as the are the OGs on this. If you google it up, you’ll find links to Friels articles on estimating TSS, including the formulas he uses with and without a power meter.
There is this calculator on the SpinLoose site, but BBW I’ve never tested it, or researched it.