A focal "Sweet spot" simply refers to the mid-point in available throw, and caters to the probability that at the extreme close-in point of a PJ's throw, where maximum Zoom is required to obtain as large an image as possible, the Lens of many PJs, not being all that good in optical quality, can reproduce focal distortion at the edges of an image. Also, at the extreme far end of a PJ's throw, an extreme loss of lumen / foot lambert performance can result.
LCD PJ usually have the widest Throw variances, DLPs the shortest. Epson PJs have excellent Lenses
that can allow for use at either end of the PJs throw potential, but still one wants to have a bit of focal Zoom reserve left. If a given screen size dictates that the Zoom / Throw distance is between 11' 6' and 24'. then placing the PJ at 12' 6" is a much better choice than 20'...............or even 16'.
But it's also nice to have such a wide choice of throw placement to consider. It's just prudent to match up Throw with screen size, reflectivity, and the PJ's available lumen output.
As for the 11' 6", I addressed that error here: http://www.avsforum.com/t/1481113/8350-and-rs-maxxmudd#post_23663275
...........explaining that I was referring to a 114" diagonal screen. I just didn't match up the change from the previously discussed 120"er.