What per cent of the seashore of Greenland is covered by Atlantic Ocean?

Given the disagreement between authorities as to the exact 'boundaries' between the various oceans, this is not going to have a definitive answer. Greenland has coast along both the Atlantic Ocean and the Arctic Ocean, but there is no precise boundary between the two.

Greenland is bordered by the Arctic Ocean to the north, the Greenland Sea to the east, the North Atlantic Ocean to the southeast, the Davis Strait to the southwest, Baffin Bay to the west, the Nares Strait and Lincoln Sea to the northwest.

https://en.wikipedia.org/wiki/Greenland

This is where it gets complicated, as we need to decide which of those listed bodies of water are part of the Atlantic Ocean, and which are Arctic. Not only do authorities differ on that as regards Davis Strait and Baffin Bay, but there is not complete agreement as to where the boundaries between these waters is. On the east, the same applies to the Greenland Sea, but there is an identifiable point on the coast where the North Atlantic as such stops and the Greenland Sea starts - Cape Nansen.

https://en.wikipedia.org/wiki/Cape_Nansen

Looking at the maps, roughly the southern third of the island is definitely on the Atlantic Ocean, and only the northern coast is definitely on the Arctic.