Does the shape of the tack depend on the shape of the intermediary items?
Yes. The shape of the created tack depends on the quality of the intermediary items and the player skill for that particular piece of tack.
Let's say you want to create horseshoes and your skill is at 30%. You need 1 flattened iron bar and the shape is at 40%.
The tack you are creating will have a shape higher than 30% but lower than 40%.