o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]
Simplistically it's related to target mach number as rb1957 suggests. However, if memory serves what's really important would be angle between tip of the cone and the lip of the duct - ie. the effective angle.