The line between the points where the two side guy wires connect to their anchors should go straight through the axis of the hinge pin at the base of the tower.

If it goes more than a little above the hinge the guy wires will loosen up as you raise the mill and tighten as you lower it.  That means if you tighten the guy wires with the mast up they can snap when you lower it, shoving the mast violently to one side and causing all sorts of havoc.

If it goes more than a little below the hinge, it means the guys will loosen as you lower the mast and tighten as you raise it.  That makes for a controlability and hinge stress issue when the mast is coming down.  It also makes for a potential snap-the-guy-wire issue when the mast is going up, though you can avoid that by leaving enough slack in the side guys that they aren't overtight when the mast reaches vertical.

The mast WILL rotate along the line between the guy anchors.  (Note that this may sweep a cone, rather than a disk, if the line is not horizontal, resulting in the mast lying at an angle other than 90 degrees from the hinge axis when it's down.)  If the hinge pin isn't parallel to that you'll put an enormous amount of stress on it - possibly breaking it and dropping the mast in an uncontrolled fashion.