Trigger milestone purchase costs from growth figures

Hi, thanks for taking the time to read my post:

I am creating a financial model for a reusable food container service for the takeaway industry.

I will need to purchase new containers every time I sign on another 20 takeaway restaurants. The restaurants grow according to some growth percentages that I want to be able to manipulate. I'm finding it hard to create a formula that doesn't require an endless list of if and floor formulae.

I have made a workaround by: making a column that = total containers required, then another column which is total containers purchased.

If the total containers purchased is less than the next month's containers required, I trigger a purcahse of more continers and add it to the running total of total containers purchased.

It's not elegant but it works. My next step is finding a way to increase the size of the purcashe order as the requirements grow month to month