Here's a simple geometry formula we use for dome settings! Use a divider caliper to measure the outside diameter of a sphere and thickness of sheet metal to be used, then multiply their difference by 1.43.

Both measurements should be in millimeters. Example: 18mm - 0.6mm = 17.4 ; 17.4 x 1.43 = 24.88mm (you can round it off to 25mm)

Source: Gemstone Settings by @anastasiayoungjewellery. Got our copy from @bookdepository! 🙌🏻