[Closed] constant size on screen

I need pflow particles to have constant size on the screen (with some randomness later) and want do to it in a script operator. not only based on camera distance but based on screen size.

I found “viewport.GetScreenScaleFactor [p3]” which is perfect, when I multiply the sizes of objects with it they are all of the same screensize, independent of the camera distance. my problem is, I don´t want to use a viewport method but calculate that probably from the camera for each particle.

Anyone have an idea for the function to get a multiplication value for that? I thought it might be similar to stereo parallax calculations, but I´m a bit stuck right now…

thanks, Julian