Heres how I went about it when starting off:
Note my lighting is 250W MH 20K Radiums on HQI setting i.e. lots of light.
1. Tested water parameters to ensure they were in the commonly accepted range required for keeping SPS.
2. Got a VERY hardy SPS like a Montipora Digitata and placed it on the bottom. After a few hours took note of polyp extension. Polyps were open and flowing in the current 'happily'. So I knew the water was ok, just as the test kits showed. Usually if something is off with the water it will be the first thing the coral reacts negatively to.
3. After about three days noticed the color was intensifying/brightening on the digi. PLaced it a little higher in the tank: about 3 inches.
4. After a few more days Coral went from pink to red and was very 'radient.' So that was my starting position. Gave me a feel for how far down the par from my bulbs reached, in this case on a 125Long three inches from the bottom easily supported a medium light requiring coral.
5. Got a green slimer and a few acros that I cant remember the name of. Placed them about 5 inches up from the bottom since they were already under high intensity bulbs at the lfs.
6. A week later the corals held color very well and polyps on the slimer were fully extended. So now I know I can start acros about 5 inches from the bottom and work their way up from there depending on lighting needs. I left the slimer at 5 inches down so I could get a lot of long vertical growth on it which I think looks cool from tanks Ive seen. Ive noticed Slimers high up in the tank seem to form differently.
So its hard to just say stick it 7 inches from the top/bottom, due to everyones lighting needs/depth of tank being different. Just know that the first thing to kill an acro will be water quality issues/parasites/battles with certain softies/ the second will be lighting issues and will take longer. I have noticed some patterns and others may chime in/argue with me:
A. Slow lightening/dulling of color=not enough light
B. Sudden bleaching= Too much light or temperature issues or SERIOUS water issues like metals, toxins, salinity or ph shock.
C. Browning/RTNing=Nutrients and or flow
D. Poor growth or RTNing form the base=not maintaining calcium/mag/trace elements