It is not quite as simple as bow waves forming and attaching at certain speeds.
The answer can be found in shock tables. When a bow shock forms (or a stood off shock - occuring in front of the intake) in will not attach to the surface if the surface is too blunt. There is no solution in the shock tables for above around 30 degrees, i.e shocks will not attach to surfaces above those angles.
It's really hard to explain, but look it up and you will see what I am going on about.