Assuming the front wing generates approximately 25-30% of the total car downforce, how much of this percentage would you expect to come from the nosecone itself, excluding the wing aerofoil elements and endplates?
Not much or maybe none at all. In fact it can also create lift.
The nosecose design's impact is found downstream of it. It affects how well the floor creates downforce.
So a better question to ask is how much does the nosecone impact the downforce of the rest of the car.