Standard EN61000-3-3 gives the limits to fulfill. All is linked with the line impedance, the amplitude and shape of the voltage drops due to the current, the pulse frequency vs the human eye perception. With his system using a standard SSR, you must generate an appropriate setting dependent algorithm to fulfill the limits. A simple duty cycle control as he is doing now is often not sufficient to comply. Another solution would be to use the SO367001. Its setting/output response is quite slow (2.5 seconds, 256 steps to get better precision) but we could make a product faster (100ms, 12 steps) provided the market is worth it. Regarding the zero cross voltage window of the SO9, it has no influence on the flicker, more about the harmonics. Its value is not precise at all, as also dependent on many parameters. In the data sheet, we put the max value with a margin.