I suggest that the emphasis of the "first team" to engage an unreached people group should not be to be a witness, evangelist or church planter. Instead, early efforts should focus on catalyzing leaders with large, strategic visions for the whole of the group, and a preference for multiplication and mobilization of additional leaders. Consider: a single team deployed among 10 million people obviously cannot reach the whole group on their own. Additional teams are needed. Consider also that, in every successful CPM/DMM analyzed, there was an outsider paired with an insider - the outsider catalyzed and assisted, encouraged, served, connected, etc. the work of the cultural insider. (Side note: I'm not talking about insider movements, but rather E-scale outsider/insider.)
I would guess "reached" threshold would clearly be found at stage 5 or 6 above. The job wouldn't be done, but it would be sustainable.
The point I'm making here: at the earliest stages, most workers should likely be focused on raising up the insider leaders and developing a multiplying strategy. Remember, just six 10X multiplications is enough to reach nearly any people group. Teams should therefore focus on how to 10X at each stage.
I'm not saying this is easy! But one thing we have to keep in mind is that multiplying is hard. When the multiplication is 2X ("each one win one") progress will be slower than if the multiplication is 3X, 5X or 10X at the same rate.
(Another way to think of the above: as stages or thresholds of progress at each stage.)