Dynamic text box "Y" position only

Yeah, I'm with Rick. You probably simply need to substitute the definition for the Y value in the generated code, but since many of us seasoned users would use their own code for such a simple thing we would need you to offer us a screenshot of whatever the script produces.

Mylenium

I don't have pins and boxes, so I do not know what the expression looks like or how the comp is set up.

sourceRectAtTimee() can use a time variable to choose when the height value is retrieved. You can also add the height + Top value from multiple text layers to control the size of the background shape.

Please show us the timeline with the modified properties of the problem layer revealed. Please embed the screenshot by dragging it to the reply field, copy and paste, or use the toolbar so we can see the image without downloading it. 

If Pins and Boxes cannot already use multiple lines of text and set time values to control when things happen, you will have to modify the expressions or write a new one. If I see the comp and the expression used to control the background, I can help.