If it says in the guide to apply the % to Sales, then you have your answer. I cannot speak to the logic without knowing how they came up with the percentage. In practice, this would be an important detail and I would expect companies with inventory to have a formal policy around this calculation. But since it is a given in your question, I say just go with it. And remember, the idea here is to not write down the inventory so much that when you do sell it you get an abnormal profit.
Sidenote: As for calcualting this “normal profit margin”, I do not think it would be just NI/Sales. Imagine if Best Buy or Amazon or Ford had to write down the value of their inventory for a specific product, They wouldn’t want to use some figure that included all products. Different products have different margins, etc…