The Diameter is the target diameter of the coil not the wire. So if you are going to wrap around a 2mm drill bit to make the coil then use 2mm in that field.
Now if you change the wire type, gauge, or resistance on page 1, the length of the wire will be automatically recalculated and sent over to the first field of page 2. It then also recalculates the number of wraps needed. The diameter field will only change when you change it based on what you are going to wrap around.
Sweet!
Very nice Work Bro!
And a damn useful App!!