how to get info for stock replenishment - past sales against inventory

[misscherylb]

hello, here's what I'd like to do:
when we need to re-order stock from our suppliers, I figure out how much we have sold over a period x (e.g. last two months), then I subtract the quantity sold from current stock level to get a sort of estimated forecast, see how much stock would be left by the next time we order, and if it's too low, we re-order that item.

Note: vm's "quantity sold" figure is not working in our system, it shows strange long-digit figures, so can't work with that.

So far I have managed to get the info by using VM Datamanager. Download "order items" of last couple of months - which involves tedious adjustments in excel, combining same products into one figure, applying sort and formulas... then when i finally have that list, i download the inventory, now have to copy this into the first excel sheet to have it all in one, but now have to adjust the rows so that the same products are in the same row.

does anyone have any idea how i could get this information easier? maybe some sql query in the database or in vm datamanager? any tip would be much appreciated!