I'm going to offer the first 50% of the book as a free preview on Smashwords.
i don't think there is any reason not to make 50% of the book available for those that want it --
HOWEVER, there is probably some real value in using a very short preview to those who want to get a flavor of it.
too often we neglect the fact that people become easily overwhelmed by information, and we are particularly vulnerable to this when we are the creators and are intimately familiar with the content.
i'd encourage you to consider having one of your proofreaders pick out a very short section (1 chapter? 5 pages?) that best captures the flavor/feel of the work, and use that small excerpt for these kinds of previews.
then if you want to make 50% or 75% of the book downloadable on your homepage, that's another matter and seems fine.