It won't work.
The angle _at the edge_ is what you have to control. An "angle guide" mounted on the spine would control that, if your face were flat. But your face is curved, in several different directions.
You have to learn to control the angle by using feedback from how the razor feels on your face as it's cutting. "30 degrees" isn't a good rule. A good rule is something like:
. . . If the razor is gliding without cutting hair, the spine needs to be raised;
. . . If the razor is scraping hair, rather than cutting hair, the spine needs to be lowered.
At least, that's how I do it . . .
Charles