I think the OP is on the right wavelength (at least to my limited intelligence) with the idea of measuring the exact time in a straight line that it takes for the chosen width opening in the curtain to travel past that line.

With a movie camera if I have a 15 degree opening traveling at 250 revolutions per second (I used high speed movie cameras to gather scientific test information) I divide 15 into 360 = 24 times 250 (frames per second) = 6000 and the inverse is my speed. 1/6000th second exposure.

You'd be doing the same thing with a laser measuring the time it takes the slit to pass by. It works the same as the adjustable angle in a rotating shutter.