U dont really need to muck around with your focusing screen. Take a bit of patience and practice and you will nail the manual focusing. Also there is a green dot focusing help at the bottom left of your screen. Aim the centre square of your viewfinder on the subject e.g. eyes, face and then turn your lens focusing ring in the direction of the arrow with the shutter button half depressed. When you see a round green dot that means the focusing for the square is perfect. Take the picture. Took me a while to get used to doing all this with my 50mm f1.2 which has such a narrow DOF haha.
Rem to use lenses that has been AI'D. Also key in your lens info into the D600 Non CPU menu so that the camera knows which lens you are using and will respond properly with the metering and picture info. I configured my front F button to allow me to change my various Non CPU lens models so that I dont have to go into all the menus to do so.