TextLineMetrics...to be continued.

OK, I am currently trying to make a text box that grows in length when the typer makes extra lines.
I tried:

if(test_txt.textHeight > test_txt.height) {
test_txt.height = (test_txt.height /(test_txt.numLines - 1)) * test_txt.numLines;
}

but text fields have some extras around the edges and stuff, so the text box got too big as time went by.

As a sidenote, I would like to mention that, if you use += and just try to add to the textfield height instead of changing the textfield height, that the first line scrolls up the text box.