CATextLayer / NSAttributedString - Determines the exact size of the displayed text

I have a CATextLayer with NSAttributedString and I want to calculate the exact size of the visible / rendered text.

I've tried this: http://www.taffysoft.com/pages/20120202-01.html

But it gives the same result for lines "1" and "5" even though with my test font (AvenirNextCondensed) the two are fuzzy but not the same.

How do I calculate the exact size of the visible text?