[wiswind]

I have never made that measurement....but I am guessing that it would not be 15 to 21......as you have noticed....it does not take a whole bunch to open the valve by sucking on it.
You could, use your Mightyvac pump.....if you have the one with a vaccum gauge on it......and pump it up to see how much vaccum it takes to pull the egr valve open.
Another check.....would be to put a "T" into the end of the vaccum line to the EGR valve......and add a small section of vaccum line to connect the EGR valve.....then run a LONG vaccum line into the passenger compartment....connected to your vaccum gauge....and go for a drive.
If you do this......I posted a couple of pictures that show a good way to run a line between the engine compartment and the passenger compartment.

I imagine the vaccum to be a fast pulsing (vaccum gauge likely will only show an average value....but good enough).