The simple crude approach is to make two assumption. One is that when sea level rises, the curvature of the Earth plays no role. Call that the "flat Earth" assumption. It's going to be a minor effect on this scale. The second assumption - a more erroneous one - is that there is no "spreading out" into new land areas as sea level rises. If you make both of those assumptions, then a crude estimate is simple. Both those assumptions produce overestimates of the actual measured sea level rise when a given volume of water is added to the oceans. Thus this crudest estimate is an over-estimate. (There are two kinds of spreading out that are being ignored. One is from the Earth curvature, and the other is from the rising oceans and seas spilling over into what's currently land.)
With those two assumptions, let SA be the current surface area of the planet's seas and oceans. Let V be the volume of new water added. Then if that causes the sea level to rise a height h, you'll have that (SA)(h) = V. Thus h = V/(SA) is the estimated sea level rise.
For your problem, knowing the ice-cover surface area isn't enough - you need to know the volume of ice that would melt and be added to the oceans. Also, you'll need to know the total surface area covered by the Earth's seas and oceans.