There are numerous ways of calculating & laying out parabolas! I have seen excel spread sheets ,layout with a T square and many others.

   I downloaded a small freeware computer program from one of the solar cooker sites that gives a quick width / depth / focal point calculation with X & Y coordinates in a text file output that can be imported onto excel or into autocad.

   Unfortunatley what I have found out is that none of the spreadsheet type output programs output a true acurate parabola!!!

   They are handy to get the rough width depth & focal point but from there I draw the parabola in autocad where I can maintain accuracy within a thousandth of an inch!

  Once I have a true parabola I draw the bows & end plates & cut a Steel template with My CNC Plasma Table & used the Templates to cut the pieces out of plywood with a jigsaw. I know I lost some accuracy cutting with the jigsaw on the prototype but all parts on the final version will be cut on the Plasma Table & should hold a tight tolerance.

   I learned how to draw a perfect parabola in autocad based on one of My most prized possesions, which is an old 1940s  drafting & sheet metal layout book.

   The book was written based on hand drafting tecniques but is a breeze to adapt to autocad using automatic guidelines & snap points.

   I use a resolution of a 1/16" of an inch to set all of the points along the parabola which for the proto type trough was 384 intersecting points & guidelines. That many guidelines makes your eyes about cross when drawing the final parabola by using the fitted curve drawing function in my cad program but it is the only accurate way I have found to generate a perfect parabola.