Perhaps this will help clarify.
For demonstration, the knife taper is 10° inclusive (5° each side).
The knife rest is angled to 20°.
This will produce a 15° bevel. To make a 20° bevel, the rest would have to be set to 25°. This is because the blade is lifted off the rest by 1/2 the inclusive taper angle of the blade.
Angle the rest at (desired bevel angle + .5 * inclusive taper angle of the blade)
Conversely, if the knife was being sharpened on the bottom side of the blade instead of the top side, 1/2 the inclusive blade taper would have to be subtracted from the rest angle.
For demonstration, the knife taper is 10° inclusive (5° each side).
The knife rest is angled to 20°.
This will produce a 15° bevel. To make a 20° bevel, the rest would have to be set to 25°. This is because the blade is lifted off the rest by 1/2 the inclusive taper angle of the blade.
Angle the rest at (desired bevel angle + .5 * inclusive taper angle of the blade)
Conversely, if the knife was being sharpened on the bottom side of the blade instead of the top side, 1/2 the inclusive blade taper would have to be subtracted from the rest angle.