I looked closely at this a while ago and have a couple pointers for you to chase if you're intent on heading down this path.  First, I would suggest reading through this document.  

http://www.ida.net/users/tetonsl/solar/page_iii.htm

They actually built a large version of what you're looking at and did a fair bit of testing concerning the focal point.  They ended up with a clever unit that you can build and will survive any number of failures in terms of the target going dry.

   Second, Mike Brown (a fellow who builds steam engines) has some calculations concerning a solar driven steam engine which show a medium sized (e.g. 9 foot diameter) solar concentrator won't create enough steam to run even a 1 hp steam engine.

http://www.mikebrownsolutions.com/solarblr.htm

   Now, there's a discrepancy between the two articles.  Mike talks about a 9 foot concentrator and the plans (from the first link) are for a 13 foot concentrator.  The 13 foot dish should be able to put out around 20,000 btu of thermal energy in unobstructed sunlight.  That's still 3-4x smaller than required by one of Mike's 1 hp steam engines.

   I'd love to build one of these homemade solar concentrators but it's so far down my list I'm sure I won't get to it for years.  Good luck with your efforts.