Machine screws won't thread into wood well.....as in the threads are too fine and close for it to be a solid connection. Hence whatever is past the insert can be any size that fits the screw as it will not make any appreciable difference.
What I would do is drill the insert size just deep enough so it bottoms out in the hole when sitting where it should. The other hole in the middle for the screw 'overflow' can be about the same size as the screw