Mr, if you intend to space the 5 equally about the circle, I think you'll find that the L- and R-fronts are too far apart. The ITU 5-speaker setup, for which virtually all multichannel recordings are made, has the 3 fronts spaced fairly narrowly--say, at 10:30, 12, and 1:30--while the 2 surrounds are spaced fairly widely--say at 4 and 8--so that they're as much sides as rears.
I suggest you experiment with your 5 988s on the floor before you attach them to your ceiling. Do observe the space to the rears of all of them, too.
Sure ought to sound great when you get them aligned!
How big is your room?
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