Spigot Form - Extrusion Help

Hi,

I am struggling with doing a form extrusion for the file attached which is a glass spigot. The spigot itself is 48.3mm in diameter from the base and as it goes up there is a curved form whilst still maintaining 48.3mm diameter. What i am wanting to know, how can this curved feature be achieved. Please see the STEP file and images attached.

Afzal