Information on tension set ring design on the internet... well, there didn't seem to be any. I looked at a lot of commercially available jewelry, and was surprised by how 95% of what was shown under tension set rings was complicated and gaudy, exactly what I wanted to avoid. I studied as much as I could from available photos of the simplest tension settings.
I turned to Machinery's Handbook for structural analysis, based on looking at the tension setting as a single loop of a torsion spring. I'm pretty sure I derived some of this from scratch on a problem set in Structural Analysis seven years ago, but I haven't done anything beyond very simple beam design since I finished undergrad. The math and my eventual results seem to line up, but it's always possible I'm totally off either with the analysis or with the actual ring dimensions.
If I'm correct, 1 pound of force at the stone setting deforms the ring about 0.007 inches. The opening was milled to dimensions that creatae about 2 pounds of force on the ring. It would take about four pounds of force to open the ring enough for the stone to slide out radially, or less to slide out along the ring finger axis.