I have the 6 foot version of that dish, I like it! got it free also. Give this program a try for finding your F/D and focal point.
hXXp://mscir.tripod.com/parabola/
Never pass up a free polar mount 6 foot mesh dish with a motor and an LNB with an LNB cover. See the dish next to my name? it is just like the one in your picture and I had to drive 1 hour to get it for free.