I’ve found it difficult to hit the 8-16-32-64 Cluster targets. Even when I come close , they don’t wind up all in one generation. The DNA is just too random. Just look at Blaine Bettinger’s cM charts to see that there are wide cM ranges for most of the cousinships. Therefore, there is not a magic threshold for any given generation.
A better plan for WTCB is to increment the cM range a little, say 5cM, and examine the new array of Clusters. Where did the Matches from the previous Clusters go? Look for cases where the Matches from one Cluster are now split between two Clusters – almost certainly these two Clusters will represent the parents of the Cluster that held all of the Matches before. As you lower the threshold incrementally, expect the Clusters formed on close Ancestors to disappear, and new Clusters to form on more distant Ancestors. Trace the Matches from Clusters that disappear to their new Clusters for very strong clues of the ancestral line. Once you are confident of the Common Ancestor (CA) of a Cluster, there are only two options for the next generation – the two parents of the Common Ancestor previously determined. And, with the lowering of the Clustering cM threshold, new Matches will be added to the mix. These new Matches have (incrementally) smaller shared segments with you, and, in general, will tend to be more distant cousins. To be sure, each new batch of Matches (as you lower the threshold each time) will probably include a range of cousinships with you. Each Cluster CA is a hypothesis, and as new evidence (new Matches) is added to the mix, everything needs to be reviewed for consistency, and discrepancies resolved. Sometimes a discrepancy is resolved by moving the Match to a correlated Cluster.
Those who try this process are encouraged to provide feedback in the Comments to this blog.
[19E] Segment-ology: Walking The Clusters Back II by Jim Bartlett 20191205