The calculations are very simple. However, I must point out, the numbers displayed on the front-end are slightly off with a few numbers depending on the frequency of certain jobs running at different intervals during the day.
In short, this is the calculation:
(Units sold / Days to count) * Days to reorder – FBA Marketplace inventory.
Note, “Days to Count” and “Days to Reorder” are customizable to your preferences. They can be viewed or modified by navigating to Settings > Replenish Rules.
For more details on replenish rules and how to set them up, please refer to this article.