From Alan Revere’s book, Professional Goldsmithing:
Disk diameter for a hemisphere = (outside diameter - wall thickness)
x 3.1415 x .92
so, if you want to make a 12 mm hemisphere from .7 mm sheet then
subtract .7 from 12 and multiply this by 3.1415 (pi) and then by .9,
divide by 2. This equals 16mm.
As Alan states in the book “the accuracy of this formula varies
according to the technique of the goldsmith applying it.” This is
because as you dap the shape it is possible to stretch the metal as
you hammer, altering the metal. Still it is as good a starting
place as you will find.
I must seriously recommend that anyone interested in goldsmithing
get Alan’s book. There is a lot of here that is both
inspiring and very handy.