How to Reduce Inventory with out using an Invoice

[grembos]

New user here,

using => PT Complete 2000 v9

If I have inventory items I want to track and count
for the purpose of calculating the COST of materials used,
but I do not sell them directly to customers, therefore,
they are never appear on an invoice, what is the best way to handle a reduction in inventory.

I think a BOM would be to involved for this environment.

The customer wants to take an inventory count first day of Q1
second inventory count first day Q2, and have PT figure out how much was used. Where would this be executed inside PT.

Thanks,
Geo