Determining the sweet spot for certain coral specimens can be challenging.  Often it is a trial and error situation.  Many factors tend to make this anything but a constant.  The type of lighting, parameters, water chemistry, and water clarity are just a few that come to mind.  This Acropora is apparently happy where it resides but this was not always true.  I tend to attach new corals to baseball size rocks so that I can reposition it if I see fit.  It is much less stressful on the coral if I can move it without touching it or gluing and removing the specimen.  This coral originally was positioned lower in the aquarium as I thought it would do best under lower lighting.  I closely observed it over the first few months and saw some recession at the base.  The pigments began to darken with little growth occurring also.  I moved it up closer to the surface where it received stronger illumination and again, observed closely for a few weeks.  I noticed some new growth at the base and the colors began to show improvement.  I believed at this point it could still use some stronger light and moved it up again.  Within a few weeks I noticed some strong growth and even a more vibrant color developing.  I call this process reading the coral.  The clues that tell me what a specific coral may desire vary but close observation on a daily basis is important.  Experience will begin to help formulate the answer if you are paying close attention to the animal.  When I am asked how much light a specific coral needs, I answer, ask the coral.