I suggest you run your purge discharge to a location which is visible. This way you can simply purge until you see white. Over purging is a waste of N20 and believe you me, it goes quick enough without wasting any. Without a thought, I almost routed mine to the side gills of the car. After a little harder thinking I decided that was not a great idea.
I extended mine with some clear aquarium type hose and ran it up to and through a small hole in the hood weatherstrip just below wiper on passenger side where it blows up in front of the passenger windshield where I can see it but it's not "in my face".
Here's a couple of pics of mine.