For those not in the know/not already using this feature, first a little introduction:
Universal soldier aiming was a feature added to the control menu of (I think it was) BF4 the first time. It is aimed, no pun intended, at unifying the perceived sensitivity across different zoom levels. In earlier titles, or when this is not enabled, you'd often notice a big jump in sensitivity when you started using scopes and it was almost impossible to get the same muscle memory when switching from a 4x to an 8x scope, to 2x ironsights et cetera. You'd have to learn everything seperately and nothing would be cohesive.
Now, universal soldier aiming does some calculations based on your aspect ratio + FOV and changes your base sensitivity on the fly to give you the same feeling with whatever zoom level you are using. It can help you become more consistent and build up a crazy muscle memory.
The feature is built around a coefficient which you can set in the control menu, just below where you enable universal soldier aiming. That coefficient is 133% by default.
EDIT: I edited this topic because some of this information is incorrect. One of the developers of this function (thanks for the support @TheNoobPolice) provided me with insights in how this function was developed and how it works exactly. For legacy value I left the original infomation in but put a strikethrough on it. Read this bold printed paragraph for my final thoughts on it.
GETTING THE MOST OUT OF UNIVERSAL SOLDIER AIMING
1. Set your hipfire sensitivity to whatever feels right to you (test by hipfire!).
2. Put all of the adv. sensitivity sliders on 100%.
3. Setting the coefficient:
If you have a 16:9, just take 133%, it's good. If you don't then:
Take 2/3's of your horizontal screen width, then divide that by your screen height. Multiply the answer by 100 and there's a nice starting value for your coefficient.
Example for 2560x1080:
2560 *0.66 = 1690.
1690/1080 = 1.56.
1.56*100 = 156%
The rest is up to experimentation because as TheNoobPolice explains, many of it is based around the perception of the user itself. Therefore it is impossible to determine the 100% right coefficient for you by pure calculation. Play an entire match switching between hipfire, irons 2x, 4x scope and 8x scope and check if the coefficient gives you the same accuracy on all zoom levels. If the high zoom scopes feel too fast, lower the coefficient. If it's too slow, increase the coefficient.
Coefficient is not a very good term for it though, because it stands for your monitor's aspect ratio.
133% => 1.33 => 4:3
Unfortunately, for most people, this standard setting will not be right as there's not many people left playing on 4:3 screens.
That means the default coefficient is wrong! Most people are using either 16:9 (1920 x 1080 and similar) or 21:9 (2560 x 1080 and similar).
To correct this do the following:
16:9 => 1.777 => 178%
21:9 => 2.333 => 233%
Dependent on what aspect ratio you have, set the coefficient to either one of those, or calculate it yourself if you have a very special screen.
One more thing: if you have changed any individual advanced sensitivity setting, I'd recommend returning everyone of them back to 100% to make sure this doesn't throw off the system. Just mark down whatever you had set it to, so you can return to it if needed
My aspect ratio is 21:9 so I put it on 233%. It took me about 20 minutes of burn in time to get use to the new feeling - after that everything was feeling so snappy and accurate, I've never experienced that before.
It might be scary to change something in sensitivity when you're already reasonably used to your settings but trust me, it's worth it! I really don't know why DICE never bothered to explain this feature correctly.