PDF to MOBI line breaks problem

Hi Folks, I'm trying to convert some books from .pdf to .mobi. I keep getting the issue of pdf line-wrap locations being preserved as line breaks (maybe you call them "carriage returns"?). I've been experimenting with the Calibre setting: PDF Input -> Line Un-Wrap Factor: The default is 0.45. Increasing that number doesn't help. Decreasing it does help, but only down to 0.20 or so. The results are much better at this setting, but still there are one or two erroneous line breaks per page.

The question: Are there any other tips for getting a better result?
Thanks All. -steve