There are so many variables, as (257NH & Mike) have said. Mine is adjustable from just over 20 to about to about 40 degrees, it has a vibration mechanism to move the material down the slope enabling a much flatter screen. It sounds like you are not going to use a vibration unit, therefore you will have to run the screen fairly steep, maybe 35 to 50 degrees, depending on how dry your material is and what type of product your going to run through it. I would make it adjustable for sure, if at all possible. JMO
If you choose to use a commercial vibration, ther a lots of them on the net, however be careful searching the internet for a vibrator
search by 12v Vibrator or pneumatic vibrator.