It seems there's a misunderstanding in the way angles are defined between a surface, its normal, and other lines.
When measuring the angle between a surface and a vertical line, you indeed consider the surface's normal line. However, if a surface is tilted at an angle of \( \theta \) from the vertical, then the angle between the surface and the vertical line is actually \( \theta \).
To clarify:
- If the surface is tilted \( 15^\circ \) from