Knowing your stock theft is vital.
The following method will tell you how to determine your figure even if you only have rough figures, you will find it a big help.
1) Do a total's report in your cash register system for 1/7/16 to 30/06/17 and tick by department. Now put this aside depending how you are setting up; it may include GST or not. If it does include GST, then take off GST by dividing by 11. If you have a department that is partly EXEMPT, you need to make an esimate for GST which you can get from your GST reports.
We will call this figure by department (RVS) Retail value sold.
2) Now you need to determine is by department.
Stock Turned Over in the books
This is simple to do.
Stock on hand at the start of last financial year which your accountant will have.
+ purchased last year again your accountant should have this.
-stock on hand at the start of this financial year, which you should have now
Now use the expected margins of each department to determine what is the retail value of this stock.
(RV) Retail value = (Stock Turned Over)/(1-Margin%) now this is EX GST, which should match your total's figures.
Your Loss = 1 -RV/RVS
Multiply by 100 and you have the percentage.
Now as a general rule, we divide this loss into four categories.
(1) Public - shoplifting
(2) Employee theft
(3) Supplier problems
(4) Internal issues- not really theft as such - accountancy errors, goods wrongly classified, goods with incorrect prices, goods discounted to clear but not put through the system as a discount.
As a first-level approximation assume each item equal although from my experience, I would say (2), (1), (4) and (3)