Used to measure time between two flashes. You have a disk with a radius line drawn (like the hand on a clock), rotating at a known speed. Take a picture exposed by the two flashes, the angle between the two lines on the disk will tell you how far the disk rotated between flashes.
Also used to measure exposure time or flash duration, if disk rotates fast enough to blur the line. The angle across which the line is blurred tells you how long the exposure or flash lasted.
Can't be used to measure shutter delay, I think, so your method is complementary, but also much easier to use!
The trick is knowing how fast the disk was rotating, but there are several tachometer designs for Arduino available on the web 
Alternatively, you could pulse some leds on the disk at known frequencies, which should give you lit dots while the flash was off, allowing you to calculate rotation speed, and then use a feature on the disk which is only visible during the flash for the rest...