MH reflector mount spacing

[liquidlunch]

I am building a new canopy and getting close to the finish line. I have always used VHO or T5 bulbs and mounting them was a no brainer.
When you mount a reflector like the Reef Optix 2(13" long), do you mount the reflector x amount of inches from the side or do you go by the bulb instead and have equal thirds between the bulbs/sides. I plan to mount (2) Reef Optix reflectors in a 48" wide tank(90 gal)

[Kaos]

I centered mine using the little glass "pod" inside the MH bulb. I figured since this was the actual part producing the light, it was best that should be centered.