The parabola will focus the rays on to ONE spot (ouch! that'll be hot!)

The equation is: y = x² / 4p, where p is the height of the focal point

Have a look at the graph x² on our little graph applet

So, for example, if you want the focal point to be 1 meter above the parabola's base you would use:

y = x² / 4

So, Width (either right or left of center) vs Height would be

W  ... H 
0.3 ... 0.02
0.6 ... 0.09
0.9 ... 0.2
1.2 ... 0.36
1.5 ... 0.56
1.8 ... 0.81

I found a really great way for you to lay out the parabola at this website: http://www.gizmology.net/parabola.htm, and after sketching using their technique, you can take a few measurements and confirm that the relationship y = x² / 4p still works.