Friend is coming to help hang lights (2 x XR15's) tomorrow. Question is how high should I hang them? BRS lists the recommended spread for this light as 18" x 18" and I have two, but that doesn't help me decide how high to hang them, right? Do I need to have a PAR meter to figure that out? Is there a standard height to hang them? The Reefer 250 tank is 21" H x 35.5" W x 20" D. Thank you for any advice!