RTFBox - Howto resize dynamically?

Hi again,
One problem solved, and instantly the next one pops up... Like:

I have a RTFBox on my Form. The user can load a File (TXT or RTF).
There is also an option to set the Font (Name, Size etc.); and, when loading an RTF, these params come with the file. So far, so good.

Now, what I need to do is to adjust the height of the RTFBox so that the lowest visible line appears fully (not only the upper portion). I don't care how much lines are visible, just so they are complete. At the moment I try to guestimate the required height by calculating the TextHeight for the corresponding font on my form and multiply this with the number of lines I want to display.

Any Ideas?
TIA