woooow. I hadn't thought about moving the light at all. That's a neat idea. ^_^
But... when I tried calculating it that way, I ended up with 6 and 2/3 feet. :/ I think it's because I used the 3 foot shadow as the base of the similar triangle. How do you know to add up the shadows? And if you add them, why don't you add up the post heights?