Answer
Cause
We assume that the question is related to the pattern rotation angle for stochastic screening.
Solution
For stochastic screening, there is usually no need in any rotation at all, that is, it is enough to make sure that the rotation is 0 for all color separations.

The situation when the pattern rotation may be needed is described below:
Suppose you have many inks in the job (for instance, CMYK + spot colors). Some areas painted by spot inks may overlap with CMYK areas or with one another. If exactly the same screening parameters are assigned for different overlapped inks, the screening will be bad. Pattern rotation may help to resolve this situation.

For instance, Magenta overlaps with Pantone Blue which will be Spot Color 0. For whatever reason we decide to use the same (Magenta) screening pattern for Pantone Blue. But since these inks overlap, we need the patterns to differ. To make them different, we can rotate the Spot Color 0 screening pattern.

However, since the patterns for Cyan, Magenta, Yellow and Black are all different, it might be enough to make sure that different patterns are assigned to the different inks in the job or there is no overlaps between the inks with the same screening patterns.

Answer
TIFF/ITflow v 8.*.0.5 and higher, applies the calibration curves to the stochastic screening. If the stochastic screening is used with the calibration curves, it is recommended to set the dot gain to low in the screening options dialog.

An optional way to calibrate the output for stochastic screening is to turn the calibration curves off and set the expected dot gain in the screening options dialog.

The reason to do that is the fact that unlike the regular screening where we can simply adjust the ink densities to control the dot sizes, calibrating the ink density for stochastic screening would just change the number of screening dots which won't compensate the dot gain properly.

That´s why we suggest that our customers take the dot gain into account by choosing one of the proper screening patterns developed for low, medium and high dot gain - select one of the dot gain radio buttons of the stochastic screening options.

The "medium" press gain screens were compensated for presses on which an uncompensated CristalRaster yields a press gain of:

Resolution, dpi	<2100	2100-3000	>3000
positive acting plates:	20%	30%	33%
negative acting plates:	33%	36%	39%

The screens for low and high press gain were compensated for presses on which an uncompensated CristalRaster yields a press gain that is 5% lower/higher than the above values.