How does precision work?

Incidentally, do you know how does < 100 precision work ?
Is this the range in squares where you have 50% to hit the target ? Is this the % to hit at some range ?
From experience, I'd say that even a Longbow of Accuracy -base prec 30, range 45- often misses at 30-40 squares.
Not to mention that even if the arrow does hit the square, it could still miss the target, especially if he has a shield.
Cheers