You get the green angle from the positions of the top and bottom of the dish.  The shape of the dish is a parabola, with the equation y squared = 4 f x, where f is the distance from the origin to the centre of the feed.  If you know the height yb at the bottom of the dish you can get xb from that equation.  The same is true for yt and xt at the top of the dish.  The angle is arctan((xt-xb)/(yt-yb).  The feed usually points close to the centre of the reflector, but perhaps a little above that to compensate for the longer distance to the top rim of the reflector compared to the bottom rim, which reduces the power density at the top of the reflector if the feed points directly at the middle.