I find the following placement of button on a suit jacket sleeve placket to be ideal:
The buttons should be 5/8 inch in diameter.
The first button should be sewn (centered) 1 ½ inches from the hem edge.
The second and succeeding buttons should be sewn ¾ inches (centered) from the first button.
All the buttons should be sewn 5/8 inch from the fold edge (or seam edge).