Try this – it might get you headed in the right direction:
maxRate = 300; //starting particles per second
decay = 1.5;
maxRate/Math.exp(time*decay)
As for MotionScript.com, I haven’t been able to get in contact with anybody at my hosting site (globat.com). In fact, their site doesn’t seem to be accessible either. So the answer to your question is – I don’t know.
Dan