In amateur work, what is the surface error upper limit you should try not to exceed on a parabolic reflector?

For a parabolic reflector to focus RF energy effectively, its surface must accurately match a true mathematical parabola. If the surface deviations are too large, the reflected waves will not arrive in phase at the focal point, severely degrading the antenna's gain and pattern. In amateur radio practice, the general rule of thumb is that the surface error should not exceed 0.1 wavelength (0.1 lambda) of the highest operating frequency. Fixed measurements like 5 mm are incorrect because the allowable physical error scales directly with the frequency and wavelength in use.