Dual Parabolic Dishes on Equatorial mount (and their descendants)

Step 3 How do we make the reflective dish? Without wasting too much material.

First of all, there are numerous ways of making a parabolic dish.
I even made one from cob about 4 years ago. I stuck the reflective material straight onto the cob.  
But normally the idea is to use lightweight  reflective material and bend and it into the dish shape. I tried petals cut to the right angles and lengths so that I could tape them together and make the shape that way.  That was not a success for me.  It tends to curve in 2 planes and that wrinkles the reflective material.  I also made a big dome mold and bent  corrugated plastic sheet over it and randomly cut  and then cut again until it fitted snugly.  then taped the lot together and that worked  reasonably well.
I thought for this one I would try something different.  A guy called Magnar owns solarcookers at ning and he has lots of information about 3 and 5 "cone" parabolic dishes.  He cuts his plastic sheet into semicircular bands, and then tapes them together to approximate a parabolic dish. 5 or 6 pieces instead of 12 or 20 petals appeals to me!
The less cuts the better! I have been studying it today and yesterday and  this is what I come up with. 
First, decide how concentrated you want your parabolic dish. Do you really need a 2 inch ball of fire?  Or will 6 or 8 inches wide do the work?   Then decide the width you want.
And then away you go.
I started under the target with my first attempt to design it.
I now think that is a mistake.  I think the best place to start is straight across from the focal point. You put a cone at 45 degrees there.
I will put explanatory diagrams in soon.
Brian.