A sluice with VDR mats is best set up at approximately 8-12 degrees (8 recommended to start). The sluice should be placed on level ground and be level across its width. The vortex riffles work with varying water flow levels but they should have enough flow over the vortices to slowly displace lighter material and the black sand from gold (about an inch of water). You should adjust the angle and flow rates to match the material being processed until your tailings checks come up nearly clean.