I just did this, on the bench, but my method should work for you, too. I used a vacuum hand pump with gauge to pull on the can. I watched to see at what vacuum the actuating rod stopped moving. My results were 16" Hg for an adjustable Moroso unit & 7" Hg for the OEM can on my V6 HEI. My tests show that the adjustment only effects the total throw of the actuator, and not amount of vacuum needed. Obviously I can't use the Moroso unit, because it operates way to high and ends up killing my economy because I'm never "all in".
To test while installed, I'd remove cap and rotor. Turn the engine by hand until you have a great view of the actuating rod and pickup. Attach hand pump and do the same test.