I am installing a Pentair TR140C which specifies 275# of 1/4 to 1/8" dia pea gravel. What I find is that, when wet, this amount just matches the height of the top of the laterals. Is this the right level, or should be laterals be covered to perhaps an inch or should they be exposed? The sand will go above this obviously, leaving the needed free board of 13.5"