I used the Fresnel laws to determine wether or not the angle 12° would affect the Reflection measurement.

If the light was polarized the difference between 12° and 0° would be 2.5%

But the spectrophotometer SolidSpec-3700 have unpolarized light, and the difference between 12° and 0° is a factor of 1.5e-5 (0.0015%).

Not enough to affect the measurement, unless the light was polarized for some reason.