Making an accurately centered cove on end grain with sandpaper was hit or miss for me.  After cutting and dry fitting the acute angle on the plank end, I clalmped the strip in a wood faced vice, cut surface up.  Using a Japan pull saw I cut a 1/16th deep groove in the center of the cut surface.  Using a triangle file, I opened the cut to a V-shaped groove.  Using the small end of a rat-tail file I opened the groove to a cove.  Sandpaper folded over a bead scrap then smoothed the cove for a tight fit.  The whole thing takes less than a minute.