Change Text Size and Keep Text Vertically Centered - actionscript-3

Question

How can I change the textSize of a dynamic text field and keep the text resting on the baseline? When I call this function and it affects the size of the text, the text gradually rises upwards within the text field as the text size decreases.
function autoFormatSize(myTextField)
{
if (! textFormat && ! textSize)
{
var textFormat:TextFormat = new TextFormat();
var textSize:int = new int();
}
textFormat.size = 50;
textSize = 50;
for (var i=0; myTextField.textWidth>myTextField.width-10; i++)
{
textSize--;
textFormat.size = textSize;
myTextField.setTextFormat(textFormat);
}
}
UPDATE
Here's a visual illustration of what I'm trying to do:

Answer 1

Use textHeight to get the height of the text. Here is how you'd position a textfield over an mc of a line:
var tfMain: TextFormat = new TextFormat();
tfMain.size = 100;
tMain.setTextFormat(tfMain);
trace(tMain.textHeight);
tMain.y = Math.ceil(mcUnderline.y - tMain.textHeight);
Use ceil to make sure it's on the x.

Answer 2

Create a Sprite and added it to stage, then align your TexField based on the Sprite. As coded below
with(bg.graphics) {
clear();
beginFill(0xeeeeee, 1);
lineStyle(1, 0x999999);
drawRect(0, 0, 200, 30);
endFill();
}
txt.x = bg.x + bg.width/2 - txt.width/2;
txt.y = bg.y + bg.height/2 - txt.height/2;

Answer 3

Considering your comment, have you thought about using autoSize instead? Something like:
var tf:TextField = yourTextField; //on timeline
tf.autoSize = TextFieldAutoSize.LEFT;
var textFormat:TextFormat = new TextFormat();
textFormat.size = 14;
var oldY:Number = tf.y + tf.height;
tf.setTextFormat(textFormat);
tf.y = oldY - tf.height;

Answer 4

To adjust your text to your text field, you have to consider a 2px gutter for each side of the text, so your function can be like this :
function autoFormatSize(myTextField)
{
var gutter:int = 2 * 2; // 4px
var textSize:int = 50;
var textFormat:TextFormat = new TextFormat();
textFormat.size = textSize;
for (var i:int = 0; myTextField.textWidth > myTextField.width - gutter; i++)
{
textSize --;
textFormat.size = textSize;
myTextField.setTextFormat(textFormat);
}
myTextField.height = myTextField.textHeight + gutter;
}
Hope that can help.