How to Calculate Diameter

We need, for example to produce some hemisphere with some diameter .
In order to fabricate a sheet metal workpiece the desired diameter, preferably to some simple calculations.

Step 1: For Example, We Make This Earring

We need to make a hemisphere with same diameter

Step 2:

Here we go, we have a formula

Step 3:

We make a calculation, it says that we need a workpiece diameter about 17mm

Step 4:

Now we have what we need. 

P.S.Sorry my english is awesome, I learning it.