I think in most cases any adjustment is there for show! It typically adjusts from rather open to far too far open, such that the b key is not closed properly.And this is the reason the front F is adjustable. A rule of thumb would be smaller opening is typically better than large opening. sounds like your horns have a typical factory set-up that should work okay for you.
However imaginative manipulations to the geometry of the area can get it where one wants.