I was struggle something similar for months, and I can let you know wha I did.
My tank is 35.4 inch wide, and I used to think a ATI hybrid (LED at center and 4 tubes each side) is enough. ATI fixture is 21 inch wide, and corals doing great so far, but I still decided adding two Kessil AP700 (already ordered) on both sides to extend my light's true coverage, which I believe it will benefit my coral for long term.
Hope it help a little...
cheers,