I suspend my 60's 2 a side on fly bars. I adjust the angle by listening to pink noise and the comb filtering then turning them in or out before I run the crank stands all the way up. I really couldn't tell you the exact angle.
Unfortunately, I haven't used that horn, so all I can do is assume.
I'd start with the splay = the horn's coverage angle, and then tweak from there - I certainly wouldn't expect to be able to close in the splay by more than (very rough generalisation here) 1/4 of the nominal coverage without ill effects though.