[xxyyzz97]

The screen frequency of different color is not the same in the same photo, this is the result of optimizing the moire free, best overprint, etc.
No idea about Founder screening.
For Harlequin, HPS, the default setting of "Y" plate is around 7% higher then the other color, in Heidelberg IS Screening is around 6% higher the the others. For others, such as Kodak (former - Scitex), Agfa, Screen also have their other optimized screening settings.