In the process of setting it up my 180g. I will have 6-60" t5's on 2 Icecap 660's centered over the tank, also I will have 2 -72" VHO actinics on a ice cap 430.
With the t5's centered the low light critters can be at the edges of the tank, you are only 6" from the edge so the lighting fall-off will not matter as I tend to prefer to have the rockwork not go all the way to the edge so that I can keep the tank side walls clear of coraline.