Writes go to disk 1 until it has 8TB free. Then disk 2 until 8TB free then 3 until 8TB then 4 then 5
After that, it does disks 1,2,3,4,5,7,8 until 4TB free
Then all the drives in order until they all have 2 TB free and so on
In your case all the 16's have less than 8TB but still more than 4 so writes go to disk 1 until its 4TB then disk 2
IE: high water is NOT 50%,25%,12.5% etc free of the individual drives capacity. It's calculated based upon the largest drive.