The 4 inch width conveyer I had in the basement 5yrs ago was at an approx. angle of 17° & I was getting some roll back of larger dirt lumps & rocks. That was the entire reason for originally building the dirt lump crusher -> the finer the dirt, the less that rolled back. The dirt I was elevating was bone dry too!!! it's almost like beach sand.
Moist dirt and crushed rock, I'm sure you could get greater than 20°.
Your belt RW doesn't seem to have much stretch & bottom drive will probably work fine for you, but you will have to run the tension tighter than if it were top drive. While you're still in the building phase, I'd recommend that you fashion some kind of brush wiper just ahead of the bottom drive roller to keep out foreign material that would affect your roller's traction or jam it up.
I have this problem of fine dirt/dust somehow getting on the inside of the belt of the conveyer that's on my dirt lump crusher, it's a bottom drive with a 5" diameter roller with a 4" width hay baler belt. Even with the conveyer tilted a wee bit, any minute amount of moist dirt that goes through the crusher, moist dirt fines will build up/jam up on one side my conveyer drive roller & stall out the crusher completely. One of these days I'll get ambitious, tear my roadway apart & take that crusher & conveyer to the shop to make some very needed improvements! There's over 200lbs of machine there caked in fine dust to monkey with.
