You can copy guide lines by Ctrl+dragging an existing one to a new location. I have mine at -120 & 120 for vertical and -90 & 90 for horizontal, the 1/3 marks. However I made a new one on accident when I was trying to copy an object, and I cannot figure out how to delete it. I have tried dragging it to the side/top as this is how it works in PPT and Photoshop and also tried selecting it and deleting it without success. Anyone know how to delete a guide?