This is what Keen uses for there 4" dredge it should be close for all and it does not heart to have the sluice box end in the water you should go by what the material in riffles look like. there should be about 3/4 of the riffle with black sand and small pebbles with larger pebbles down stream from the riffle with a small patch of expose carpet in front of the next riffle it should look like that after you stop running material and let the water run for a while
SLUICE BOX ADJUSTMENT
Most models have a slight adjustment to raise or lower the sluice box. The proper sluice box adjustment can effect the
recovery of values. If the sluice does not have enough angle, the sluice box will "load up" causing the riffle openings to
fill with unwanted excess material. Too much angle will cause the material to flow too fast, resulting in loss of values, evidenced
by the riffles running too clean. The optimum adjustment of a properly working sluice box is evident by only a
portion of the riffle is visible while operating. A loss of values can also occur if the solid content of the suction discharge is
too heavy in solid content. Remember, the solid content should not exceed 10 %. A normal sluice box tilt is approximately
3/4” inch to the running foot. Afour foot sluice box should have an approximate tilt of 3"