You draw an imaginary triangle, call it the image triangle. The base of the triangle is that portion of the image that correlates to your subject’s physique. In other what is the length in millimeters of the image of the face. You can count pixels, and if the size and spacing (from spec. sheet) is known you can derive this value. This distance is the length of the base of image triangle.
The height of this triangle is the focal length of the lens. You can now draw this triangle image triangle with pencil and paper. You can use a protractor or trigonometry to find the size of the angles in degrees.
Find the ratio of height to base. Say the focal length was 80mm and the face height covered 8mm. Find the ratio 8 ÷ 80 = 0.10.
Now we compute the object triangle. It will be a larger triangle however all angles will be identical to the image triangle. In other words they are similar triangles.
The base of the object triangle is an imaginary line, center of camera lens to subject. Say this subject distance is 10 feet = 120 inches. The face of the subject measures 120 x 0.10 = 12 inches. (a ratio is dimensionless).