You can just drag the edges of the loop up on the timeline above the event. If you zoom way in, you can trim the loop down to the millisecond. If you play in looped mode, you can hear the results of your changes interactively.
I know that in sf 6 if you click on the "auto snap to zero" option ("options" pull down menu) It will snap your selection to the nearest zero point & works pretty well MOST of the time - its a great "lazy man's" approach to loop making.