Just in case you haven't seen it, here's the new website for viaNavigo! It explains very well how it all works. It was in beta the last time I looked, thought you might find it interesting! Thank you for this glorious website! It's very helpful!
https://www.vianavigo.com/en/tickets-and-fares/subscription/the-pass-navigo-semaine/
(scroll down to #2)