The best way I've discovered to test shutter speeds of stuff like that is to attach the shutter to your large format camera body (or the mounted lens, however you intend to use it), and take a digital camera with manual controls and place it inside the camera, preferably near where the film is inserted. . Then what you do is set the camera for a long exposure, say 5-30 seconds, with the self timer activated, trip the self timer, and close up the LF camera box the digital camera is in. Now wait for the self timer to quite beeping, letting you know it's starting the 5-30 second exposure. During that 5-30 second exposure, trip the shutter you made or are testing. Wait out the rest of the 5-30 seconds. What you should now have is a digital exposure with a known ISO, and a known aperture. From there, you remove the digital camera from the box and take a series of photos of whatever you were trying to photograph before, with the same ISO and aperture, only now you're using a series of known times. Compare the known times with the unknown times, and where the two photos look identical, you now know your shutter speed. Test it again a few more times, this time setting the aperture or ISO to get a better exposure during the test, and with some different variables (like a difference scene or different light source).
It would be a lot easier to explain with a video or some diagrams. It sounds difficult and time consuming, but it really only takes like 5 minutes. And the results are very accurate (assuming your digital camera is accurate), and doesn't require any math or special tools (other than the camera).
Or you could build a shutter speed tester. They're simple to make, so long as you have a computer to hook it up to and some basic soldering skills.