Ideally, rocks shouldn't touch the glass, though sometimes, that isn't always feasible in order to create a 'scape. Also create a structure that is as open as possible. If you are planning on keeping corals that grow significantly, then make a lower structure. You'll also want to place the rock down first, and add sand afterward to improve the stability of the structure. Last but not least, use the rule of thirds (or a rough golden mean) to create a structure that is pleasing to the eye. So in other words, make a structure that occupies some multiple of a third. That could be one third from the side, full tank, split a third of the length of the tank, etc. Tons of possibilities. In most cases, slightly less rock is necessary.