How can I adjust the availability of variants based on sales of other variants in the same product?

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I sell cookies. It's a small business and production numbers are limited.

For each type of cookie I sell, I may offer the following variants:
Variant 1: Boxes of 4 cookies
Variant 2: Boxes of 20 cookies

The problem is this: say I have 30 individual cookies to sell this week and of those 30 individual cookies a customer has selected 3 boxes of 4 cookies (12 cookies total) and put them in their shopping cart. This scenario would bring my total inventory of individual cookies down to only 18. Variant 2 requires 20 individual cookies and as such, could no longer be sold. 

I would like a way to automate the number of each variant that is available for sale based on the number of individual items within those variants that have already been selected for the shopping cart.

Is there a way that I can specify the total amount of individual cookies available, offset it against the number of individual cookies that are "bundled" within each variant of the product and then automatically update variant inventory numbers accordingly?

All help points to bundles apps but I don't see how these can help me, as I am not selling individual cookies. The product itself is the box of 4 cookies and the box of 20.