Karmic debt is not calculated. It is noticed. If a two-digit total of thirteen, fourteen, sixteen or nineteen appears anywhere in the chart before the final reduction, the doctrine holds that a debt is being carried, and the position that produced it is read accordingly.
The obvious question is why those four. The sources this atlas cites for the chart assert the list rather than derive it, and the explanations offered afterwards are unconvincing on their own terms. Thirteen is said to be the debt of laziness, fourteen of excess, sixteen of pride, nineteen of the misuse of power. Nothing in the arithmetic distinguishes these four totals from the other two-digit sums that reduce to the same digits, and the doctrine does not claim it does.
Thirteen carries an obvious cultural charge in the societies where this system was written, and it is hard to read its inclusion as independent of that. Sixteen and nineteen have no comparable folk reputation in English-speaking culture, so borrowing does not account for the whole list.
What can be said with confidence is that the layer is a later addition. It presupposes the reduction procedure and the two-digit intermediate stage, so it cannot predate the positions it comments on, and it belongs to the same instinct that produced the master numbers and the compound: a rescue of information the reduction had discarded.
The atlas records the doctrine and the four numbers because the tradition holds them, and records that no reason for the selection has been given.