Setting the vertical size of a Sprite object equal to the vertical size of a TextMesh?

I'm trying to create a background color for a TextMesh (to make the text easier to read against the main game background) which involves re-sizing a separate white gameobject/sprite to fit the size of the TextMesh.

I seem to be able to get the horizontal/x size correct but for some reason the vertical/y size never matches up with the vertical size of the TextMesh; it's always too high or too low, seemingly never equal to the bottom-most part of the TextMesh. (I assume there's something wrong in the calculations or a better way to do this.)

Shown on the attached picture, I would like the white background to line up with the bottom of the text.

I've experimented with various code but this is what I currently have:

1. public void ShowDialog()
2. {
3. ...
4. Vector3 punkTalkPosNew = new Vector3(punkDialog.transform.position.x, punkDialog.transform.position.y, 0);
5. 
   
6. dialogBackground.transform.position = punkTalkPosNew;
8. dialogBackground.transform.localScale = new Vector3(punkDialog.renderer.bounds.extents.x,
9. punkDialog.renderer.bounds.size.y, 0);
10. ...
11. }

​

Originally I was just drawing the text in Photoshop and have it show up as Sprites; should I stick with that instead? Or what's the best way to go about this? Thanks.

(PS I'd post this on http://answers.unity3d.com/ to avoid bothering forum-ers but my past questions have never received any responses there.)

 

I guess I'll stick to using text created in Photoshop (for now) since I know how to do that, but I'd still appreciate somebody trying to explain what's happening here.