How to calculate WEEKLY OR DAYLY COVERAGE IN INVENTORY

Hi all

i'm becaming crazy...i've no idea how to calculate the coverage for my material.
Let me explain what's coverage:

if we open the year with 100 of an item and plan to sell
50 in January
30 in February
40 in March

then I have sufficient in inventory at that point in time to cover all of January requirement (so 1 month cover) plus all of February requirement (so 2 months cover and half of March requirement (so 0.5 months cover). All up 100 in inventory provides 2.5 months cover.

How can i this in excel...for all months and lot of products?

Thanks
Paolo