As a general rule, you will have more even coverage and higher penetration if you hang your lights higher vs lower. Also, hanging lights to have overlapping canopy coverage would be better vs non-overlapping hanging arrangement. Of course, if you are height limited then you must make due with the growing environment you have to work with. When height is restricted try to use a higher quantity of lower power units and arrange them to provide even coverage at the highest possible height your room allows.