Not sure I'm getting mechanics right

[thejeff]

There's no randomness to see if the missile deviates.
However:
:quote: The game will randomly determine whether the missiles deviate long or short, left or right, or some combination. The actual distribution is a bell curve -- most projectiles will fall within the middle of the deviation range, but some will land at the extremes.

I would assume that the peak of the bell curve is actually the target square. Your missile can deviate, but can deviate 0 squares. Counterintuitive, but makes sense mechanicly. (And matches observed results. No noticeable holes in arrow spread patterns.)

I'd also correct my previous assumption. The manual is inconsistent about max deviation. When the example calculates the deviation of 10, it specifically says, "maximum deviation is 8 x 1.25, or 10 squares" ignoring the division by Precision mentioned in the text. I'd assume the example is wrong, since deviations greater than the range don't match what I've seen in the game.