PAR38 mounting height vs. lens spread for 7.5G rimless cube

[dgmanny]

Hi, I recently purchased a 7.5g 12x12x12 cube and I am planning to use a single par30/38 lamp for lighting. I was wondering if anyone has some real life experience on how far above the tank a par lamp would typically be placed with 40 degree, 60 degree and 80 degree spread. I would like to keep the lamp 6-12" above the tank.

I am looking at the rapidled, exocitata and boost led lamps.

Thanks,