In the period from 15 days onwards, there is no discernable logic to their allocation of seats to the D bucket. Most of the time there is no allocation at all now of course, regardless of how few J seats they've sold, but if there are D bucket seats available on any of those days, there are always 7 of them.

In this time period they usually only seem to make D seats available about 1 day per week, and there is never any correlation with the number of unsold J seats. I've seen flights with 80% unsold J seats and nothing in the D bucket, while there are 7 D bucket seats in a 60% unsold cabin a day or 2 earlier or later.