Some weeks ago I was wondering about something like that.
I searched the web and after some time analyzing different mathematic formulas and lots of martian things (to me) like those, I change my view of the problem and get a solution. I can?t give it directly to you, but I can tell you (cuz mine is a very concrete example and is not adaptable for other projects) the way of getting it (very basic but it works).
The base is use the "three rule" ("regla de tres" in spanish, I don?t know how to translate this :S).
Knowing the factors you may know (power, speed, or whatever) make test depending on your game resolutions, and once the bullets hits a static point, use the "three rule" to get a custom formula that works in your concrete game resolution (sorry for my bad english).
The idea is that: is the bullets hits a certain static (at first) target with your desired factors, extend it to make it works.
You cant see what I got some weeks ago (taken directly from the devlog):
<img src="http://egoitzosa.com/x/wildWest/2012_12_08_flechas.gif" border="0" />