Mobile Flickering Fingerprint on Touch

Hi

Whenever I make a too big light importance volume mobile I get a flickering on my Galaxy S4, whenever I do touch input on the virtual joysticks.

The flickering resembles my fingerprint when touching the screen.

Now I know that light importance volumes should be as tight and as small as possible, even more so or mobile games. But is there a actual way to tell in advance how big the volume can be, so that this be taken into consideration in the process of level design? So that one doesn’t end up with a level that is to big to be covered by the volume? Or what should I do here?