So don't calculate, OBG. Use something alreday parabolic like the inside-of-an-umbrella projects. I bet as a practical matter you can figure out where curved relector panels focus their light / heat against a target like a piece of cardboard just like you zero in with a magnifying lens.

Or do as I am: start with a cardboard box project instead of one that is parabola-based. My thought is to then duplicate the box with thin aluminum sheets lined with foil at first, then with super reflective material like:

http://home.att.net/~cleardomesolar/solareflexpanels.html