The infrared photo transistor/led used in the final refinement has a 10 angle of view. The one from the old mouse most likely has a similar angle of view.

I do all testing with the lens removed. It makes little difference if the sensor is at the center of the shutter opening or if its center is at the smallest marked aperture opening of the lens.
The photo transistor/led has a rise and fall time of .1ms. The rise time is the time delay from when the sensor is first sees light and starts to conduct to when it reaches the maximum peak for the amount of light reaching it. The fall time is the time the sensor continues to conduct once the light source has been removed.

Now my thinking or logic might be flawed here, but it seems to me on an iris shutter, the shutter is open for less time on the edges of the round opening than the center, right? The shutter takes time to open and close, nothwithstanding the time it stays open. The edges are the last to be un-covered, and the first to be re-covered, right?. So if I make my tester with the phototransistor just a couple inches away from the rear lens element, it seems like I'd get a false reading. Seems like the phototransistor would have to be at the film plane to get a true reading.
A leaf shutter is a dual aperture device with one aperture manually set via a lever according to a calibrated scale and the other a continuously variable from full closed to full open and back to full closed via a controlled timing system.
When a leaf shutter is set (cocked) high spring tension is locked onto a lever that when released will try to force the shutter blades ( continuously variable aperture) fully closed but is locked in the set position by another lever. When the shutter is tripped the shutter blade controller is released and thrown open by a weaker spring with no opposition to the blade controller moving except for its and the blades pivot pins resistance. The shutter blades go from full closed to full open in .15 to .3 milliseconds. When the shutter blades reach full open the blade controller releases the locking lever allowing the main setting lever to try to close the shutter blades via the blade controller. The main setting lever encounters resistance to closing from the delay timing gear train.

Setting a speed testing sensor close to the outer edge of the shutter will give false shutter times. Setting the speed testing sensor at the smallest marked aperture edge will give more accurate fast speed times.

Testing is normally done with the aperture wide open and the sensor at the center of the shutter opening or lens center. By the time the shutter blades have closed down to be effectively 2 stops smaller than the smallest aperture the lens is marked for the resulting extra exposure the film receives is insignificant on a properly working shutter. A installed lens may cause false shutter test speeds unless a very bright light source is used for testing. I use a minimag flashlight with fresh batteries set to spot at 2 inches held just above the shutter opening. I have the top edge of the photo sensor near center of a piece of 1/4 inch foam core and the foam core is rested against the rear edge of the shutter. Increasing the distance to or from the shutter blades only affects the intensity of the test light source needed for testing. (click on the chart to download for printing)