There would not be a minimum angle, although there should be enough of an angle to help keep the axle centered.. think of a seat of a swingset hanging straight down with the chains parallel, this would be somewhat easy to shift side to side. If you pulled the tops of the chains together or apart the seat would stay centered much more than if the chains were straight.
The degree will be based on width at the batwing, width at the frame and the length of the bars, these companies have set degrees because of specific popular setups ; 11 degree would be specific to a 32-34 Ford, with maybe a 22" c to c bar
7-9 degree would be maybe a Model A, depending on axle width with about a 24" c to c bar
we use a 3 degree on the front (frame side) for our 4- link and radius rods, but we use a 51" bar on a 26.5 frame.