Ai ext bracket question

[chrisl1]

I am in the process of building my dsa 105 rimless. It measures 48x24x18. I just purchased the AI ext 48" mounting brackets and the 6" top extender bracket where the lights mount to. My question is is the 6 inch bracket enough or is the 12" what I need to center it. The bracket is mounted behind the tank so it comes from back to front. I don't have the lights yet and probably won't till after Christmas and the holidays (3 kids) $$$$$ lol anyone that has the AI hydra 52 with any thoughts on this?