The amount of sugar you add depends on what you are using this icing for. For piping fine lines, as for borders to be filed with more icing, the consistency should be fairly thick, like frosting. If needs to hold it's shape without being so stiff that it's difficult to squeeze from a pastry bag. For glazing large areas (flooding), the icing should be just pourable, so that when spread, the surface flattens out. If you're using the same icing for both purposes, make the thicker icing first, pipe the borders, then add a few drops of water at a time to thin the icing before pouring or spooning it into a pastry bag for flooding.