1. On purchasing data: I am excluding it for the sake of simplicity. And not all products are bought every month.
2. On using 400 instead of 472: The reason why I used "400" (note that this is a manual input) is because there will be stock takes at the end of every month, and sometimes there will be discrepancies in the inventory. So my team decided that we should have a manual input of inventory at the end of each month.
So the objective is to take the latest inventory data available and then perform further forecasting, month-on-month.
I hope you can tell from icwiener's post that it really isn't so straightforward. The measure you suggested will not work for future months where there is no value for the inventory. that's why I'm trying to figure out a measure to calculate the forecasted inventory value.