SourceRectAtTime not working

The layer scale is not taken into consideration. Make sure that you have not inadvertently scaled the layer. That will foul it up, but it can be compensated for. 

Try adding these expressions to the appropriate properties of a rectangle that is just below a text layer:

//Contents/Rectangle 1/Rectangle Path 1/Size;
src=thisComp.layer(index - 1);
ref = sRc.sourceRectAtTime();
refScale = sRc.scale * .01;
x = ref.width;
y = ref.height;

[x * refScale[0], y * refScale[1]]

//Contents/Rectangle 1/Rectangle Path 1/Position;
src=thisComp.layer(index - 1);
box = src.sourceRectAtTime();
refScale = src.scale * .01;
x = box.width / 2;
y = box.height / 2;
t = box.top;
l = box.left;

[(x + l) * refScale[0], (y + t) * refScale[1]]

//Shape Layer/Transform/Position;
src=thisComp.layer(index - 1);
src.position;

You have to make sure all of the other shape layer properties are at their default position.

The shape layer will perfectly line up with the text layer no matter what the paragraph justification is and no matter where the text layer is in the comp as long as the anchor point of the text layer has not been fiddled with.

Impossible to know without any actual info about your font formatting and other details. That said, of course there will be minor discrepancies due to quantization errors. Even on large text some internal font values will end up as tiny fractions like 0.00001 em and wil lsimply be rounded up or down since ultimately the engine has to make a determination on whether a pixel is visible or not. As suggested by the cactus, sinmply figure this into your planning and also don't mess too much with custom kerning, scaling character widths and so on to minimize those artifacts. Also find alternate fonts. Many fonts these days have specific screen-optimized variants that avoid a lot of the issues just like generally better balnced, commercial fonts may work better than stock system fonts or free fonts from the Internet.

Mylenium

My guess is After Effects is calculating the width, but it can't take into account exact character width.  How about adding a correction value to the text width, something:

w = s.width + s.width*0.01