This is a very debated subject by the way.
There has been no official post regarding how hit calculations are made for ranged, so people are forced to do their own research, and come to conclusions based on them (shocking, I know).
I think everyone agrees with you needing +5% hit to hit a equal level target.
However, when a mob is higher level than you, you'll see different theories.
Some say 1% per level (Basically level difference).
Some say 0.2% per level (0.04% hit modifier per skill difference, 1 level gives 5 skill levels)
Some say it's not a flat increase.
So what we have here is a spread, if you ask how much +hit is needed to always (barring the slight chance to miss you never can remove) hit, you can get multiple answers.
8% (5% + 1% per level difference)
5.6% (5% + 0.04% per skill difference)
8.6% (5% + 1% per level + 0.04% per skill difference)
13% (non-linear)
17% (non-linear)
That's a pretty big difference.
Personally, I think it's 5.6%, based mostly on my own hits / misses and a post from allakhazam, which might be an interesting read (editing this in here after I post). I might be wrong though, which is why I'm still looking for better proof than what I currently have. This is mostly due to my spreadsheet, which, I'm guessing, some of you know by know
However, using only tests, you can get very different results, even after a long period of time, since it's a random system. We do not have access to a controlled enviroment to test as accurately as would be preferred.
I don't think I can give you a better answer than this
This is discussion I'm pretty interested in, so please share any information you might have, including links and tests =)
Edit: As promised - http://wow.allakhazam.com/db/class.h...89813325357946