[aerohead]

Chaz,it's not really about angles at all but rather an angle which occurs at a particular point along a roofline or the flanks of a vehicle.
For a short-bed truck,the angle measured at the back of a cap which will support attached flow might only be 12-degrees.
A long-bed truck would be able to support a steeper angle due to it's increased length.
For both short and long-bed truck,no angle may work,unless a gentle transition of curvature is present upstream of the "angle".
Two aerodynamic templates are available for you to use at the aero forum.
The first can be found at "Permanent Kammback for Metro" and the second at "Aerodynamic Streamlining Template".
From either,you will see that the roof curvature begins almost in perceptibly at first,building greater degree of curvature the further down field.
Without the curve,your flow will likely separate immediately,with little benefit to you.
The science of pickup truck airflow is well established now since 1988 thanks to the folks at Texas Tech University and the SAE Paper they published.No guesswork is necessary today.You can copy what they've done and go about your business with a working cap or you can ignore the science and modify at your own peril.