Just so happens, I purchased one when it went on sale at Christmas. I need to look at the flash metering, I didn't know you could use it for that.
Watch this. I already have it set to start at the time you need so you can bypass the 2 minutes of blah blah blah.
https://youtu.be/J5GqVTYuT-Y?t=2m10s
Another way you can set the exposure, if you know you need to work with a given speed or aperture...
Scenario: I want to shoot at an aperture of f/4
1. Go to where the subject will be.
2. Set camera to Aperture priority (for this scenario).
3. Make sure you have no exposure compensation.
4. Take a picture with the Expodisc over the lens pointed to where you will be shooting from.
5. Review the file of the gray image and look to see what the shutter speed is. You now have the shutter speed needed for the aperture and ISO you have dialed in.
6. Bonus - if you don't change your white balance, the gray image can be used in Photoshop Camera Raw to set white balance
You can change this up if you needed a specific shutter speed and aperture is not the priority. There is a whole combination of things you can do.
Hope this helps.