I had a 24" Photon V2+ over my new 36x30x20 tank. Side to side it's OK. Not perfect as the extreme ends are a bit dim, but it works OK and looks OK.
My problem was the 30" front to back size. If I pulled the fixture forward to reduce shadows at the front, the back of the tank was too dim. If I moved it back, the whole tank would be lit OK, but I had more shadow under the front rocks than I liked.
I solved the problem by getting a 32" Photon V2+ and set it behind the 24" Photon. The front one runs a bit more white at midday and the back one a bit more blue. With the 30" distance front to back, the blue behind the white makes the tank look even more than 30".