wall foam

When installing wall foam, do I go from the bottom up? My foam is not the same height as the pool wall. Most of the foam that is sold is 48" and my walls are 52". Do I leave the small gap at the top to be hidden under the top rail? Or do I make it flush at the top and use the pre-made foam coving to cover the gap at the bottom. I am installing blue board as my base. It seems to me that the wall foam should come down to the blue board base and the pre-made coving cover where the two meet. I just wanted to make sure.