Sharing a quick and simple way to calculate the tz from a light source. You can also use Delete SOP to remove all points from Trace SOP.
Keep only 1 point to play with, trail it and then use it for Particle. The tracking is stable, better lower your phone screen brightness.
In the .zip you may find the .toe file
In case you don't have a camera there's a sample for video input, very useful during day time!
I've seen recently many users painting with flash lights.
Hope someone will find it useful!