How much variance for your raw ingredients are acceptable and how much is too bad, so you need to pay attention, try to understand and investigate.
Human beings are not robots. Everyone makes mistakes. Even if your chefs are always using weights, it is impossible to make end month inventory 100% accurate. There is always going to be either shortages or excess.
Although common F&B acceptable variance standards are 5-8%, you still have to consider how you use this or that ingredient. For example, if you only sell beer by bottle, even a 1% variance is reason enough to ask your bartender questions. As for meat and fish, a 10% variance can be acceptable since trimming of fats, fins, tail and head can significantly vary from fish to fish. However, if you receive fish fillets the variance should be much less as compared to a whole fish.
How do you count the 5 or 10%? Take the total usage per month and compare with the variance. For example if you sell 35kg of Sea-bass and get a 2kg difference at the end of the month, you can consider this as a normal deviation since it is a little bit more than 5%. Make sure you don't have a constant shortage or excess every month. If you do, then you probably need to revise your standard wastage percentage.