Kintesh- I don't think you'll find an exact formula either.
The issue is that the dished surface is not what is called "developable". Such a surface is one that can't be unrolled, unfolded, etc, to a flat surface. Contrast this with a circular cylinder or a cone, these can be unrolled to a flat surface; meaning they can be "developed". Trying to flatten out a non-developable surface will impose strains in it (conversely, forming a flat plate of metal into a dished head induces strains...and stresses...in the plate).
The result is that there is no simple formula based strictly on geometry to determine the diameter of the blank required for a formed head.
However, as the material properties are known I imagine that it is possible to determine a formula to determine the blank dia. This would require knowledge of the material's properties: modulus of elasticity, yield stress, ultimate stress, Poisson's ratio, and possibly more properties as well. Thus the blank's diameter would vary slightly depending upon the material. I have never seen such a formula, nor heard of anyone discussing the possibility of using the material properties to determine this. It's likely that head manufacturers have worked out rules of thumb that inherently (or explicitly) account for these properties.