To Taper a Face(s) of a Solid by a Keyed-in Angle

Select the Taper Face Feature tool.
From the Draft Direction option menu, choose the required direction setting.
If required, turn on Add Smooth Faces.
In the Draft Angle field, type the required value.
Select the solid at the point from which you want the taper constructed (relatively).
A dynamic arrow indicates the location of the identification point and the direction of the taper.
Select the face to be tapered.
(Optional) Using <ctrl+ data point>, select further faces to be tapered.
Accept to view the proposed taper.
Accept the taper.

Select the solid (1) and an arrow displays the relative point from which the draft will be constructed, and the Draft Direction. | Select the face to taper (2).

Accept (3) to view the taper and again (4) to complete. | Resulting taper with Add Smooth Faces off (left) and on (right).

Left: Taper with positive angle. Right: Taper with negative angle.