A while ago, I searched the archives of Orchid for a ratio for
making toggle clasps. I found much helpful but no math.
The subject came up with a fellow metalsmith friend, who said, "oh,
that's easy, I figured it out." She says you measure the inside
diameter of the circle part of the toggle, and the bar part needs to
be twice as long as that measurement.
Metalsmith, Certified PMC Instructor
Hard to Find Tools for Metal Clay