How do I have a cell look to another cell and formulate a number based on
inventory levels? For example:

Column A = 1800
Column B = 100
If column B is less than A how many lots of 1800 it would take to replenish
to level back above the 1800. In this calculation I would need (1) lot of
1800. My balance would be 18100. Now if I were negative 600 for column A it
would take (2) lots of 1800 to replenish my stock.