[aerohead]

From the work on bellypans published in Hucho's text,2-1/2 degrees is reported to be the ideal angle for the upswept diffuser.If you'll measure the "run" of your proposed diffuser,from origin to termination,and multiply that by the sin of 2-1/2 degrees,it will spit out the proper "rise" for the panel.For,say,a 36-inch long diffuser,the vertical component would be just a little over an inch and a half ( for a meter,it would be about 38-mm ).
Or you can draw it on the floor of the shop with a protractor and measure it directly.