Calculate number of lines in text box

[RobinReumers]

I currently have a big text field that I fill with a script. When I change the text box on the layout, it can be that one line will jump over and split into 2 (if over 70+ characters), now I would like to find out in exactly how many lines it is split.

I could say if it's higher than 70 characters: 2, higher than 140: 3, … but this is not very foolproof as it could depend on the words used in the text (breaking at a "space"). Is there any foolproof way that I could use for this calculation?