You cannot take 70% for the life of the client. More like 15-30%, and it will be very hard to track.
For example, what if they upsell to the client on an Enterprise package, why would they let you know? You need to have a very good relationship with the developers to go that route. And you will need to take only a reasonable cut (15-30 top).
The easiest way is to have them pay you one time fee per conversion. That fee can be worth 3 months worth of subscription or something along those lines, here you don't have to deal with tracking all of these subscriptions and more..