If the bevel angle on most razors is, let's say 18degrees. then the incidence angle with the spine against the face would be half that: 9 degrees.

So, half a spine width (i.e. spine laying flat against skin) is 9 degrees. The full spinewidth is 18 degrees.

I would therefore say that the already inclusive 9 degrees on the spine against the skin, coupled with a single spine width above that gets you to 27 degreees incidence angle of cutting axis to your face.

So, in actuality, a single spine width above the skin IS the mystical 30 degree incidence angle!