Parts inventory: the minimum that keeps a job moving
Stock per location, goods in and goods out, and a notice when a part drops under the minimum.
The shelf is not a memory
Promising a part because someone thinks it is there stops a job halfway. Before you say it is in stock, someone has to be able to read the quantity. If only one person knows, the shop stops when they are out.
Each item needs a minimum. Below that number, a notice appears before the part is missing on an order. The shop sets the minimum from what it actually uses, not from a generic catalog.
Goods in and goods out
Record goods in when the order arrives and goods out when the part goes to the job. If the count is corrected only now and then, it cannot be trusted within a few days.
Do not assume the quantity drops by itself when an order closes. Confirm the outgoing line when the part leaves the shelf. A part marked used that is still in the box throws off the next estimate.
Per location
Stock at one location is not stock at the other. A single combined number makes the shop promise a part it cannot fit today. Each location has its own quantity and its own minimum.
If the part is at the other site, the transfer has to be visible. Until it arrives, it is not here. Telling the customer you have it when it is across town is a half promise.
Start with what stops cars
Parts, consumables, and tools should not share the same mental drawer. A few categories are enough at the start. You do not need a deep catalog tree.
Begin with filters, pads, oil, and whatever you order every week. Add the rest when it shows up on a job. A perfect empty inventory does not help the counter.