At shutter speed settings shorter than 1/30 or perhaps 1/60 this is of very limited merit. These are the reasons why.
(1)
It will tell you absolutely nothing about the consistency of exposure across the whole film gate. The Leica type shutter is an excellent design, but it is no more immune to tapering than any other type of classic focal plane shutter. You can, however realistically measure longer exposure times with reasonable accuracy using this method.
(2)
Given that times faster than 1/20, 1/30 or 1/50 (varies depending on the exact shutter) are the product of the travel speed of the curtains and the distance between their adjacent edges—even if an accurate measurement of the curtain transit times is obtained, the dimensions of the slit are an unknown quantity. What I mean by this, is that unless the escapement is engaged for the speed being set, the curtains transit the gate at the same velocity at every exposure setting, whether it's Eg 1/1000 or 1/100. Yes, the second curtain will commence moving sooner and sooner after the first, as the set speed is shortened. But even measuring the variation in second curtain release timing will not directly inform the actual amount of exposure. (See above, it's the curtain speed and slit width which determine actual exposure, and the slit dimension is not constant across the gate).
In the real world, the challenge with these vintage cameras isn't usually that a curtain doesn't run. It's that it is running erratically (Ie not releasing at the correct moment, not accelerating to spec, dragging due to old lube, stiffened curtains, foreign matter in the mechanism etc). And even if the timing mechanism does release a second curtain to spec, the only way to really have any idea of what exposure the curtains are effecting across a whole negative (short of—obviously—exposing some negatives) is to either inspect the slit visually (which is how Leitz used to do it, back in the day) or, measure the exposure across the gate electronically.
I realise this is the internet, and we're all experts on everything. So why should you heed my comments? Well, obviously—you don't have to. I will, however mention that I've spent a lot of time working on shutters over the last fifteen years so that I could use the cameras involved. Including a few Leicas among other classic fp shutter designs. In the last few weeks, for example, I've attended to two vintage Leicas which had lain idle for decades (a 1946 IIIc and a 1942 IIIc). Each of their shutters are now working very well. This image from the 1942 was made at 1/1000 (Pan F Plus, ID-11, 1950 Summitar). The speed with the tightest slit and hence—by far—the most sensitive to erratic curtain behaviour. As you may see, I've calibrated the shutter to produce consistent exposure right across the film gate (look to the concrete at lower edge, in particular). So, I actually do have a half a clue about what I discuss above.
Regards,
Brett
Edit:
Comments in the image caption as posted at the following link are not without relevance to this discussion so rather than pasting them into this post, you may (if desired) read some elaboration about shutter behaviour here:
www dot flickr dot com/photos/tasmania_film_photography/55111212751/in/dateposted/
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