One of the problems with a leaf shutter is acceleration and deacceleration of the shutter blades. Obviously, the shutter cannot open or close instantaneously because it has mass. If you look at a graph of a leaf shutter opening and closing, it ramps open (slopes up) had a flat open period, and then ramps down to close.

The exposure period is the total time from the beginning of the edge of the ramp until the trailing edge of the closing ramp. Lenses with leaf shutters have a built in delay (for electronic flash - X-sync) to time the flash to compensate somewhat for the shutter not being completely open when the flash is triggered.

Here's the problem. The faster the shutter speed you are using with a flash, the greater the exposure problem as leaf shutters are notorious for being inaccurate as they near the peak shutter speeds. Also, the larger the shutter, the greater the problem because of greater mass, etc.

My advice is to expose at 1/60th second or less using an electronic flash with a leaf shutter because you actually control the light reaching the film with the aperature setting (in most cases) - and the shutters are far more accurate in speed at lower shutter speed settings.

If you use fast shutter speeds 1/125, 1/250, 1/500, you may find yourself using a larger f/stop or higher flash power setting to compensate for under exposure caused by the shutter open/close cycle.