There's really nothing more to it than practice unfortunately. I've been playing PC shooters since the original Doom and thought I would never get into console shooters, but after playing a bit of CoD4, I started getting into it and after a year or so of playing, I felt as in control playing with a controller as I did with the keyboard / mouse combo.
One thing though, you will never be as accurate using a controller, it's just impossible. Whereas using a keyboard and mouse in a shooter could be described as "point and click", with a controller, I've found that it's more like sweeping towards the target and picking the right moment to press fire in that sweep.
It sounds kinda strange, but it can be 90% as effective as keyboard / mouse control once you get used to it.
"I always have problem to properly calibrate my mouse when playing FPS games (Combat Arms)."
I know this answer is very late; but I've played quite a bit of Combat Arms over the years and I believe it's critical to OP's (former) issues.
Combat Arms in 2011 was using an engine similar to what is now available as "Combat Arms: Classic" (this is not the same as the one available on Steam called "Combat Arms:Reloaded"; which doesn't have these issues). In the old engine, there are a couple things to note about mouse movement.
Combat Arms had forced mouse smoothing (I can't find the reference for this anymore, it was said by a developer on Nexon forums when they were working on CA:Reloaded). The best way to help avoid major effects from that was to put a cap on the FPS that you could maintain constantly.
"My mouse is Razer Copperhead."
Razer Copperhead, like many gaming mice, has a polling rate up to 1000 Hz. Combat Arms doesn't like that. The aim will zig zag and not follow straight paths if the polling rate is above 125 Hz. Most gaming mice have settings to limit the polling rate in their software.
TL:DR The Combat Arms: Classic (or Combat Arms in 2011) engine does not work well for calibrating mice in general. The game has forced mouse smoothing so using fixed FPS is suggested. It also requires a polling rate of 125 Hz or below to allow the mouse to move in straight lines.
Update: If (for some reason) you do still want to play Combat Arms Classic with a 1000Hz mouse, I have found the workaround for that as well.
In your Combat Arms Classic folder there are two .exe's for the game. Engine and Engine2. Engine2 is actually a "beta" build that has been included with classic for awhile.
The Engine2.exe actually works to run the game, (and removes forced mouse smoothing) as long as the game is up to date (which means you need to run the normal launcher to update the game).
You'll want to make a shortcut for Engine2.exe. Then right-click on the shortcut and go to properties. Where it says Target, add -windowtitle to the end of the target. It should look something like:
"C:\Program Files (x86)\CombatArms Classic\Engine2.exe" -windowtitle
This shortcut will launch the game with the build which removes the forced mouse smoothing.
Best Answer
Mouse movement interpretation can be vastly different between different game-engines, and often even between different games using the same engine. There is no standard for mouse sensitivity, so the only way to measure it is to use our own impression.
When I start a new FPS game, the first thing I do to calibrate the mouse sensitivity is:
Instead of a 90° turn you can also perform a flick-movement between two points.
I could also imagine a third, more exact but more technical method.
Games use different FOV (field of view) angles. Maybe you feel better with calibrating your mouse sensitivity for the FOV and not for the game world. In that case replace the 90° turn with aiming at a point and moving your mouse until the point you aimed at is exactly at the border of your screen.