Printing to pdf

[Peter Haworth]

Following along with the lesson at
http://lessons.livecode.com/m/4071/l/29177-how-to-create-pdfs-using-livecode.
All is the same as the lesson except adjusted the print rectangle to fit
the card I'm printing.  Running on OSX 10.10 and LC8 DP11, using Preview to
display the pdf.  Font is Arial, size 10.

The field on my card shows all the text correctly with each line wrapped
appropriately.  The pdf cuts off the last few characters on each line.

If I reduce the width of the print rectangle by approx 50 pixels, the pdf
is created correctly.

Why is this adjustment necessary and is it a case of trial and error to get
the correct width or is there a formula that will yield it?