Shadow mask settings for me look different per game resolution and per host PC resolution (differing amounts of vertical moire type patterns etc.). So it seems to be impossible to get them "right". As a result I just put the mask_alpha to 0 and go with 50% vanilla scanlines with defocus as a compromise.
Is there a guide or default that explains all these settings and how to get them "right"? hlsl.txt isn't of much help.