T line vs A line when action is to spin thoughts in our head

Hi! I realized that oftentimes in my model the A line is filled with thoughts that I spin in my head. But then, how do you know what thought to put in the T line and what thoughts are included in the spinning thoughts of the A line? I would guess the thought that triggers the other one goes in the T line, but it is a bit of chicken and egg situation… I guess that in the end it doesn’t really matter that much?  Thanks!