Glenn,
I once asked Shore Sails in RI that question, and was told that they didn't
really know..... In the end I calculated what load the tack ring could take,
and used that plus 50% as a basis. Size the tack line to be the first thing to
give, for a little extra piece of mind.
As a guide, I have used 2500lbs on a 40-footer, and 5000 lbs on an Open 60.
Tack fittings still in place. ;-)

Steve