I did an experiment.
I want to build a path for the cattle feed for Hungry Cattle. I wanted to know the minimum angle the rice path should have (from horizontal) so that the rice would flow reliably.
I set up the experiment shown above. To start with the inclined plane (which is a 3D printed wolf in PLA) was level. I placed some rice on the end, then raised it until all of the rice slid to the bottom. I measured the amount I raised the plane then calculated the angle with some trig. I repeated the experiment a few times and took the worst case.
I measured 30 degrees from the horizontal. So my design will use 35 degrees.
Wikipedia tells me that this angle is called the angle of repose.