After Effects Expression to track x pos of longest text line

In your example, if Red Solid 1- Layer 2 is below the text layer and has the anchor point in the default middle of the layer, and the text layer is Right Justified with the baseline shift of zero, this expression will position the square exactly at the top left corner of the text layer.

ref = thisComp.layer(index-1);
box = ref.sourceRectAtTime();
pos = ref.position;
xOfst = width/2 - box.left;
yOfst = box.height;
[pos[0] - xOfst, pos[1] - yOfst]

 If you want to offset the Red Solid 1 layer by 10 pixels to the right and move it up 5 pixels, modify the last line of the expression to add the padding like this:

ref = thisComp.layer(index-1);
box = ref.sourceRectAtTime();
pos = ref.position;
xOfst = width/2 - box.left;
yOfst = box.height;
ofst = [-5, -5];
[pos[0] - xOfst, pos[1] - yOfst] + ofst;

If you have multiple lines of text and you want the Red Solid 1 layer to stay lined up with the left edge of multiple lines of text, you'll need to add a Math Max function to look at the left edge of multiple layers like this: (the example is for two lines of text) ;

ref = thisComp.layer(index-1);
box = ref.sourceRectAtTime();
ref2 = thisComp.layer(index-2);
box2 = ref2.sourceRectAtTime();

xOfst1 = width/2 - box.left;
xofst2  = width/2 - box2.left;
xOfst = Math.max(xOfst1, xofst2);

pos1 = ref.position;
pos2 = ref2.position;
posX= Math.max(pos1[0], pos2[0])
posY = ref.position[1];
yOfst = box.height;
ofst = [-5, -5];
[posX- xOfst, posY - yOfst] + ofst;

You have separated dimensions in your sample comp. I almost never do that because you can't edit the motion paths. If you want different motion paths or the position of the red box to attach itself to the longest line of text, you can throw those calculations into the expression.

These expressions will lock the Red Solid 1 layer to the position of the top text layer, and the bottom text layer must have the same X position values. The expression offered by Adam does not lock the layer to the text layer's position.

I hope this helps. 

Oh, I see now that you aren't trying to track multiple lines in a single text layer. Please disregard the project I posted, which is meant for a much more complicated task.