For the tablet settings, you're going to want to set the mapping to how big you play osu in, if you play fullscreen, set it to fullscreen, if your go windowed, set it to how where and how big the window is.
Inside of Osu, if your going to be dragging and using a keyboard, your going to want to disable mouse buttons. When you drag the pen acts as if the mouse is pressed so it'll mess up your gameplay.