CAD plans the working stage, too
The prong you see in the model is not necessarily the prong you will see on the finished piece. If you design only the final appearance, you may not leave enough material for the work and the setting.
The stone and the head set the relationships
The relationship between girdle, prong and head structure changes between stone shapes and between designs. A number detached from the surrounding geometry means nothing.
Stone shape changes the consideration
Pear and marquise shapes may need a more protective solution at the tip. With a princess or emerald cut, the prong style can also be a design decision that shapes the character of the piece.
And in a halo, think about all of it together
The center prongs have to be coordinated with the neighbouring stones and the metal between them, to avoid interference in setting and keep a clean look.