You want to retain some material in your sluice, the space between the large riffles should show some carpet.
In other words if its too flat, the riffles will be full of material and won't be able to catch the gold.
If it is too steep, the riffles won't work either as all the material blows out the end.
Experiment until you find an angle that holds some material behind the riffle, but isn't totally full from riffle to riffle.
Get some old lead tire weights, smash them flat, cut into small pieces, and throw them into the material you're running to see if you catch them. Good Luck. post some pics.