Originally Posted by krokdylz
Or you match it like x cm on your mousepad is y cm movement on your screen in every game.
That's pretty much the only way to do it. Compare/match the on screen movement by dragging the mouse from one side of the the pad to the other.
When you move your mouse form one side of the pad to the other in CS: GO the cursor ends up in say the middle of the screen or where ever. The you do the same in LoL and adjust the sensitivity until the cursor lands in the same place. If you are having a hard time eye balling it use something like blu-tack to mark the spot temporarily (careful with what you choice to use as you don't want to leave marks on the monitor).
But really like others have said, why would you even want to do this as what feels right for CS: GO is not going to be the optimal speed for LoL. Even within FPS games the best speed varies form game to game.