Right, 2.5 inches. Of course, you can cheat those a tad. You've got it. Though you can go out another three or four spaces to the right and no one will hold it against you. Joe jots down several notes on a 3 x 5 card, studies the card for a moment, then scribbles another note.
Some writers center all the characters' names.
Personally, I don't think it matters. The look of the script pages is slightly different in either case. A page break appeared in the middle of my dialogue. You use MORE at the bottom of the page to show that the character's dialogue continues onto the next page. Then CONT'D next to the character to show that it has roots in the previous page.