Forecasting check

Hi guys!

I have this issue that has been wrecking my head for the last few months, I was wondering if anyone can help or suggest something...

Background info: Started working for a big import company that doesnt use software so all the work and planning has to be done via excel.
Take a look at the file attached, it's an example of my actual file. It's very simplified obviously - imagine thousands of SKUs and tens of delivery dates.

What I want to do is come up with a way to calculate if the stock is sufficient between deliveries. Example: If I have 2 months stock and my next delivery is in 1 month, I'm obviously covered. In case of sales increase in the current month, I can tell if I can survive the next delivery using the "months in stock" value. However , I need to manually check every time whether the already future planned deliveries (delivery 2, delivery 3 and so on) will be sufficient to keep me going.
I am trying to use conditional formatting, but I am struggling with it.

The principle is :
For delivery1 : In stock/avg sales = months in stock. If months in stock > months till delivery1, all good. If months in stock < months till delivery1, highlight somehow.
For delivery2 : if (Instock+delivery1)/avg sales > months till delivery2 all good. If < , highlight
For delivery3 : if (Instock+delivery1+delivery2)/avg sales > months till delivery3 all good. If < , highlight .

Writing this just gave me a massive headache, I hope I managed to explain my troubles.

Any ideas would be much appreciated.