The classic definition of "unreached" - "a people group lacking a church able to evangelize the group to its borders without cross cultural assistance" - lacked any way of measuring when the defined goal was reached. Depending on which missiologist you talk to, this is either a feature or a bug.
Various groups, including the Joshua Project, measure "reachedness" in terms of 5% Christian and/or 2% evangelical. (JP uses "or"; the IMB uses "and"; and this minor difference alone can create vastly different lists for reasons I leave to the reader to consider as a thought exercise.) The World Christian Database uses Worlds A, B and C: World A, less than 50% evangelized; World B, over 50% evangelized but less than 60% Christian; World C, over 60% Christian. The latter brings interesting nuance because it mixes the concepts of evangelization and conversion, bringing out the empty space in between; but some have faulted it because it counts all Christians as Christian.
To me, there is no "hard and fast" line at which a people group is either "reached" or "evangelized" (depending on which definition you wish to use); I am also of the theological persuasion that there is no hard and fast line of conversion - many people have trusted Christ long before they ever "prayed a prayer" or were baptized. There is a mystery to it, a personal nature to it, and thus a continuum. In fact, I might trust Christ with my soul far more after walking with him a few years than I did at the point of conversion or baptism; in my opinion we make a lot of conversion and not enough of transformation and discipleship, but that's a topic for another day.
Of course, what we really want to do is measure the remaining task: at best, to figure out where we need to send resources; at worse, to figure out if we're done and we can go play. In both cases, we essentially want to know if, for any given place, the task is done.
In an earlier post I suggested measuring places according to a simple scale. To determine whether a group is reached, I'm thinking about a similar approach using density of churches and current growth rates. The question of "is this place reached" breaks down into three components: (1) is there a church that can do the task, (2) is the church "doing the task" (and the best correlation, at the moment, seems to be whether the church is growing), and (3) is the task done, measured by whether the church is of a sufficient size.
It seems to me there are three thresholds to be considered:
-3 Massive, exponential, multiplying decline: ex. Iraq
-2 Declining, faster than any population decline: ex. Central Asia, Germany
-1 Declining, but slower than any population decline: ex. Egypt
0 Stagnant, little or no growth or decline, probably in context of population stagnation
+1 Growing, but not as fast as the population: Russia
+2 Growing, faster than the population: India
+3 Rapid multiplication/growth: China
The benefit of such a system: it doesn't just tell you the location of a country on the continuum, it also tells you the direction it's headed. If your headed to your house, and your house is north of you, you need to know both where you are on the map--and which direction you're going. I'm in the process of putting this scale on the District Survey, starting with countries and then moving to provinces. Once I get the newest version of the mapping software (my current version of AtlasGIS is crashing under Windows 10), I'll map it as well. In the meantime, Patrons can watch the Patron Dropbox Folder; I'll add some more spreadsheet PDFs soon there. (You can become a Patron and get access to the Vault here.)
In the meantime, you can do this kind of research yourself, using whatever list you favor. Just build these columns: country name, population size, population growth rate, size of the church, ratio of church to population, and church growth rate. Country names, population sizes and growth can be gotten from the UN population data set. Church sizes nationally can be gotten from Joshua Project. Church sizes, and more importantly, growth rate can be gotten from Operation World.