I was lucky enough to get heroic percision on my Poisoneer Prophit, so I got him a black bow (and some other goodies) and sent him out on an assassination spree
His precision is now 28! Without the addition from the bow...
Lets just say he hits just about every time (though doens't always damage obviously...). I guess my point is that if you can arrange these 1 on 1 situtations you can probably get a better feel for how precision works.
Now... I just need to empower him in death once so he can summon up a decent screen of longdead or soulless and then go to town with his bow...