markc mouse fix is basically to make sure there is no acceleration when the windows sensitivity is being used for a game. Quake Live by default is set to "in_mouse 2" which is raw input, so you don't really need the mouse fix for quake. If you want to use the mouse fix to set up acceleration you need epp checked. But if you don't want to set up a custom acceleration with marc mouse fix then uncheck epp to turn of windows acceleration. If you prefer to use windows input the command is "in_mouse -1". Again these commands are for quake live. Most games are going to be raw input or windows input and you probably won't be able to set it. So it always good to just use the markc mousefix. But i would recommend disabling enhance pointer precision by unchecking the box. And no it does not need to be checked.