Thanks for your suggestion.
We use our own formula to calculate when an item goes in "low stock". You can read about it here.
If what you want is just to modify the number of days to reorder on that formula, you can do so from ATUM Settings >> Stock Central >> Days to reorder.
However, if you need to use your own formula for the low stock calculation, what we can do is to add a filter to the
\Atum\Inc\Helpers::is_product_low_stock() helper, so you can are able to do so within a custom hook.
As this is pretty quick, I've just added it there (before returning the value) and will be available on the next ATUM version.
The new hook is this:
apply_filters( 'atum/is_product_low_stock', $is_low_stock, $product )