Turning Ev3 precisely has been a problem. Do you use a mat for running EV3 on. Can you tell me on what material to get the mat printed?
Mats are sometimes use with Mindstorms to create line following bots or for defining more complex tasks.
I can't give great advice on printing a mat for EV3, as I've never done this, but I would imaging that you'd want to be able to print on a large, fairly stiff piece of paper or cardboard. If you are just trying to create a basic line for your robot to follow, you can use a piece of posterboard and then add a line with black tape, Sharpie, paint, etc.
There are several different mats that LEGO has made available for Mindstorms over the years:
I believe the most recent is actually the box sleeve in the EV3 kit:
You might want to just purchase of the officially released mats.